Do a sync after reading this post then go to about:sync-log and copy the latest error-sync-*.txt file's content in a new reply.

1501155423960 Sync.LogManager DEBUG Flushing file log 1501155423964 browserwindow.syncui DEBUG observed: weave:ui:sync:error 1501155423965 browserwindow.syncui DEBUG loginFailed has sync state=success.login 1501155423967 browserwindow.syncui DEBUG loginFailed has sync state=success.login 1501155424000 Sync.LogManager DEBUG Log cleanup threshold time: 1500291424000 1501155424037 Sync.LogManager DEBUG Done deleting files. 1501155425535 browserwindow.syncui DEBUG loginFailed has sync state=success.login 1501155425549 browserwindow.syncui DEBUG loginFailed has sync state=success.login 1501156023954 Sync.Service DEBUG User-Agent: Firefox/54.0.1 (Windows NT 10.0; WOW64) FxSync/1.56.0.20170628075643.desktop 1501156023955 Sync.Service INFO Starting sync at 2017-07-27 17:17:03 in browser session Aa1su_ki1t6m 1501156023956 browserwindow.syncui DEBUG observed: weave:service:sync:start 1501156023957 browserwindow.syncui DEBUG onActivityStart 1501156023960 Sync.SyncScheduler DEBUG Clearing sync triggers and the global score. 1501156023962 Sync.Telemetry TRACE observed weave:service:sync:start 1501156023963 Sync.Status INFO Resetting Status. 1501156023963 Sync.Status DEBUG Status.service: error.sync.failed => success.status_ok 1501156023965 Sync.BrowserIDManager DEBUG _ensureValidToken already has one 1501156023973 browserwindow.syncui DEBUG loginFailed has sync state=success.login 1501156023982 browserwindow.syncui DEBUG loginFailed has sync state=success.login 1501156025580 Sync.SyncScheduler DEBUG Next sync in 600000 ms. 1501156046071 Sync.Resource WARN GET request to https://sync-460-us-west-2.sync.services.mozilla.com/1.5/81541648/info/collections failed: [Exception... "NS_ERROR_UNKNOWN_HOST" nsresult: "0x804b001e (NS_ERROR_UNKNOWN_HOST)" location: "JS frame :: resource://services-sync/resource.js :: Channel_onStopRequest :: line 509" data: no] Stack trace: Channel_onStopRequest()@resource://services-sync/resource.js:509 < waitForSyncCallback()@resource://services-common/async.js:98 < Res__request()@resource://services-sync/resource.js:386 < Res_get()@resource://services-sync/resource.js:415 < _fetchInfo()@resource://services-sync/service.js:500 < sync()@resource://services-sync/stages/enginesync.js:82 < onNotify()@resource://services-sync/service.js:1115 < WrappedNotify()@resource://services-sync/util.js:164 < WrappedLock()@resource://services-sync/util.js:120 < _lockedSync()@resource://services-sync/service.js:1105 < sync/<()@resource://services-sync/service.js:1097 < WrappedCatch()@resource://services-sync/util.js:95 < sync()@resource://services-sync/service.js:1086 1501156046080 Sync.Status DEBUG Status.sync: success.sync => error.login.reason.network 1501156046080 Sync.Status DEBUG Status.service: success.status_ok => error.sync.failed 1501156046081 browserwindow.syncui DEBUG observed: weave:service:sync:error 1501156046081 browserwindow.syncui DEBUG onActivityStop 1501156046084 Sync.SyncScheduler DEBUG Next sync in 600000 ms. 1501156046085 Sync.ErrorHandler ERROR Sync encountered an error: Error: NS_ERROR_UNKNOWN_HOST (resource://services-sync/resource.js:395:19) JS Stack trace: waitForSyncCallback@async.js:98:7 < Res__request@resource.js:386:14 < Res_get@resource.js:415:12 < _fetchInfo@service.js:500:14 < sync@enginesync.js:82:16 < onNotify@service.js:1115:7 < WrappedNotify@util.js:164:21 < WrappedLock@util.js:120:16 < _lockedSync@service.js:1105:12 < sync/<@service.js:1097:7 < WrappedCatch@util.js:95:16 < sync@service.js:1086:5 < Res_get@resource.js:415:12 < _fetchInfo@service.js:500:14 < sync@enginesync.js:82:16 < onNotify@service.js:1115:7 < WrappedNotify@util.js:164:21 < WrappedLock@util.js:120:16 < _lockedSync@service.js:1105:12 < sync/<@service.js:1097:7 < WrappedCatch@util.js:95:16 < sync@service.js:1086:5 1501156046090 Sync.Telemetry TRACE observed weave:service:sync:error 1501156046096 Sync.Service DEBUG Exception calling anonymous function: Error: NS_ERROR_UNKNOWN_HOST (resource://services-sync/resource.js:395:19) JS Stack trace: waitForSyncCallback@async.js:98:7 < Res__request@resource.js:386:14 < Res_get@resource.js:415:12 < _fetchInfo@service.js:500:14 < sync@enginesync.js:82:16 < onNotify@service.js:1115:7 < WrappedNotify@util.js:164:21 < WrappedLock@util.js:120:16 < _lockedSync@service.js:1105:12 < sync/<@service.js:1097:7 < WrappedCatch@util.js:95:16 < sync@service.js:1086:5 < Res_get@resource.js:415:12 < _fetchInfo@service.js:500:14 < sync@enginesync.js:82:16 < onNotify@service.js:1115:7 < WrappedNotify@util.js:164:21 < WrappedLock@util.js:120:16 < _lockedSync@service.js:1105:12 < sync/<@service.js:1097:7 < WrappedCatch@util.js:95:16 < sync@service.js:1086:5 1501156046109 Sync.ErrorHandler DEBUG Addons installed: 0 1501156046111 browserwindow.syncui DEBUG loginFailed has sync state=success.login

On the new device try limiting Sync to just Bookmarks and (say) Passwords for the initial Sync event. If that is successful, then add Tabs, Preferences and maybe History. If that works too, then see if Addons will Sync.

Thank you. However, no change. No log event was created after that change and trial. The latest log 1501161352419 Sync.Service INFOLoading Weave 1.56.0 1501161352422 Sync.Engine.ClientsDEBUG Engine initialized 1501161352424 Sync.Engine.ClientsDEBUG Resetting clients last sync time 1501161352431 Sync.Engine.BookmarksDEBUG Engine initialized 1501161352434 Sync.Engine.FormsDEBUGEngine initialized 1501161352438 Sync.Engine.HistoryDEBUGEngine initialized 1501161352441 Sync.Engine.PasswordsDEBUG Engine initialized 1501161352443 Sync.Engine.Prefs DEBUGEngine initialized 1501161352447 Sync.Engine.Tabs DEBUG Engine initialized 1501161352447 Sync.Engine.Tabs DEBUG Resetting tabs last sync time 1501161352451 Sync.Engine.Addons DEBUG Engine initialized 1501161352453 Sync.Engine.Extension-Storage DEBUG Engine initialized 1501161352454 Sync.Service INFO Mozilla/5.0 (Windows NT 10.0; WOW64; rv:54.0) Gecko/20100101 Firefox/54.0 1501161352455 Sync.SyncScheduler DEBUG Clearing sync triggers and the global score. 1501161352456 Sync.Status DEBUG Status.service: success.status_ok => success.status_ok 1501161352456 Sync.Status DEBUG Status.service: success.status_ok => success.status_ok 1501161352456 Sync.AddonsReconcilerINFORegistering as Add-on Manager listener. 1501161352456 Sync.AddonsReconcilerDEBUG Adding change listener. 1501161352457 Sync.Tracker.HistoryINFOAdding Places observer. 1501161352487 Sync.Tracker.BookmarksDEBUGmigrateOldEntries: Missing bookmarks tracker file; skipping migration 1501161352488 browserwindow.syncui DEBUG observed: weave:service:ready 1501161352489 Sync.Status DEBUG Status.service: success.status_ok => success.status_ok 1501161352491 browserwindow.syncui DEBUG loginFailed has sync state=success.login 1501161352492 browserwindow.syncui DEBUG loginFailed has sync state=success.login 1501161356496 Sync.Status DEBUG Status.service: success.status_ok => success.status_ok 1501161356500 Sync.Service DEBUG User-Agent: Firefox/54.0.1 (Windows NT 10.0; WOW64) FxSync/1.56.0.20170628075643.desktop 1501161356503 Sync.Service INFO Starting sync at 2017-07-27 18:45:56 in browser session pwZKHC-0bM0j 1501161356504 Sync.Service DEBUG In sync: should login. 1501161356505 browserwindow.syncui DEBUG observed: weave:service:login:start 1501161356508 Sync.Status DEBUG Status.service: success.status_ok => success.status_ok 1501161356508 Sync.Service INFO Logging in the user. 1501161356517 Sync.BrowserIDManager INFO Waiting for user to be verified. 1501161356517 FirefoxAccounts DEBUG already verified 1501161356518 browserwindow.syncui DEBUG loginFailed has sync state=success.login 1501161356522 Sync.BrowserIDManager INFO Starting fetch for key bundle. 1501161356524 FirefoxAccounts DEBUG already verified 1501161356527 Sync.BrowserIDManager INFO Getting an assertion from: https://token.services.mozilla.com/1.0/sync/1.5 1501161356527 FirefoxAccounts DEBUG enter getAssertion() 1501161356561 browserwindow.syncui DEBUG loginFailed has sync state=success.login 1501161356568 FirefoxAccounts DEBUG got keyPair 1501161356568 FirefoxAccounts DEBUG getCertificateSigned: true true 1501161356574 Services.Common.RESTRequest TRACE local time, offset: 1501161356574, 0 1501161356576 Services.Common.RESTRequest TRACE hawk auth header: Hawk id="49f3b45c72e1023688963704315df809b392b98ae9657c7d067795bfa2b9a8d2", ts="1501161356", nonce="gmJHD16wimA=", hash="lZ+yVko1LrJuLBicbXOUAryeOoUgPmSOXHKc5TodDiI=", mac="UhRcxIAIhI6oa2RdKEwXQd89l4UdCl2rrUNvEaQ11ns=" 1501161356577 Services.Common.RESTRequest DEBUG POST request to https://api.accounts.firefox.com/v1/certificate/sign 1501161356577 Services.Common.RESTRequest TRACE HTTP Header authorization: ***** (suppressed) 1501161356577 Services.Common.RESTRequest TRACE HTTP Header content-type: application/json 1501161356577 Services.Common.RESTRequest TRACE HTTP Header accept-language: en-US, en 1501161356577 Services.Common.RESTRequest DEBUG POST Length: 888 1501161356577 Services.Common.RESTRequest TRACE POST Body: {"publicKey":{"algorithm":"DS","y":"946A60800D5A242F3C45D32FB6301E36EE7163B79DF942DC9B51769193A33888D65EB792D717B889A9CB56307001FD03319EA47A06FC6E1E31EE3FE0C784223650AB323050EFB5840104D153CDCCA2DADD85FA7687A46C5BA99642251473A57EDE97E54F7C39ADC14F5B186BCFA02E220D4EF2AE9ACD058BC07BD01862702077","p":"FF600483DB6ABFC5B45EAB78594B3533D550D9F1BF2A992A7A8DAA6DC34F8045AD4E6E0C429D334EEEAAEFD7E23D4810BE00E4CC1492CBA325BA81FF2D5A5B305A8D17EB3BF4A06A349D392E00D329744A5179380344E82A18C47933438F891E22AEEF812D69C8F75E326CB70EA000C3F776DFDBD604638C2EF717FC26D02E17","q":"E21E04F911D1ED7991008ECAAB3BF775984309C3","g":"C52A4A0FF3B7E61FDF1867CE84138369A6154F4AFA92966E3C827E25CFA6CF508B90E5DE419E1337E07A2E9E2A3CD5DEA704D175F8EBF6AF397D69E110B96AFB17C7A03259329E4829B0D03BBC7896B15B4ADE53E130858CC34D96269AA89041F409136C7242A38895C9D5BCCAD4F389AF1D7A4BD1398BD072DFFA896233397A"},"duration":21600000} 1501161387062 Services.Common.RESTRequest TRACE onStartRequest: GET https://profile.accounts.firefox.com/v1/profile 1501161387063 Services.Common.RESTRequest TRACE Channel for GET https://profile.accounts.firefox.com/v1/profile returned status code 2152398861 1501161387064 Services.Common.RESTRequest DEBUG GET https://profile.accounts.firefox.com/v1/profile failed: 2152398861 - NS_ERROR_CONNECTION_REFUSED 1501161387068 FirefoxAccounts ERROR Background refresh of profile failed: {"name":"FxAccountsProfileClientError","code":null,"errno":998,"error":"NETWORK_ERROR","message":"[Exception... \"NS_ERROR_CONNECTION_REFUSED\" nsresult: \"0x804b000d (NS_ERROR_CONNECTION_REFUSED)\" location: \"JS frame :: resource://services-common/rest.js :: onStopRequest :: line 483\" data: no]"} 1501161387078 Services.Common.RESTRequest TRACE onStartRequest: POST https://api.accounts.firefox.com/v1/certificate/sign 1501161387079 Services.Common.RESTRequest TRACE Channel for POST https://api.accounts.firefox.com/v1/certificate/sign returned status code 2152398861 1501161387079 Services.Common.RESTRequest DEBUG POST https://api.accounts.firefox.com/v1/certificate/sign failed: 2152398861 - NS_ERROR_CONNECTION_REFUSED 1501161387080 Hawk WARN hawk request error: [Exception... "NS_ERROR_CONNECTION_REFUSED" nsresult: "0x804b000d (NS_ERROR_CONNECTION_REFUSED)" location: "JS frame :: resource://services-common/rest.js :: onStopRequest :: line 483" data: no] Stack trace: onStopRequest()@resource://services-common/rest.js:483 < waitForSyncCallback()@resource://services-common/async.js:98 < makeSpinningCallback/callback.wait()@resource://services-common/async.js:168 < onNotify()@resource://services-sync/service.js:844 < WrappedNotify()@resource://services-sync/util.js:164 < WrappedLock()@resource://services-sync/util.js:120 < WrappedCatch()@resource://services-sync/util.js:95 < login()@resource://services-sync/service.js:860 < sync/<()@resource://services-sync/service.js:1090 < WrappedCatch()@resource://services-sync/util.js:95 < sync()@resource://services-sync/service.js:1086 1501161387082 Hawk DEBUG (Response) /certificate/sign: code: null - Status text: null 1501161387084 FirefoxAccounts ERROR error POSTing /certificate/sign: {"error":{},"errorString":"[Exception... \"NS_ERROR_CONNECTION_REFUSED\" nsresult: \"0x804b000d (NS_ERROR_CONNECTION_REFUSED)\" location: \"JS frame :: resource://services-common/rest.js :: onStopRequest :: line 483\" data: no]","message":null,"code":null,"errno":null} 1501161387084 FirefoxAccounts ERROR HAWK.signCertificate error: {"error":{},"errorString":"[Exception... \"NS_ERROR_CONNECTION_REFUSED\" nsresult: \"0x804b000d (NS_ERROR_CONNECTION_REFUSED)\" location: \"JS frame :: resource://services-common/rest.js :: onStopRequest :: line 483\" data: no]","message":null,"code":null,"errno":null} 1501161387088 Sync.BrowserIDManager ERROR Non-authentication error in _fetchTokenForUser: {"error":{},"errorString":"[Exception... \"NS_ERROR_CONNECTION_REFUSED\" nsresult: \"0x804b000d (NS_ERROR_CONNECTION_REFUSED)\" location: \"JS frame :: resource://services-common/rest.js :: onStopRequest :: line 483\" data: no]","message":null,"code":null,"errno":null} 1501161387088 Sync.Status DEBUG Status.login: success.login => error.login.reason.network 1501161387088 Sync.Status DEBUG Status.service: success.status_ok => error.login.failed 1501161387088 Sync.BrowserIDManager ERROR Background fetch for key bundle failed: {"error":{},"errorString":"[Exception... \"NS_ERROR_CONNECTION_REFUSED\" nsresult: \"0x804b000d (NS_ERROR_CONNECTION_REFUSED)\" location: \"JS frame :: resource://services-common/rest.js :: onStopRequest :: line 483\" data: no]","message":null,"code":null,"errno":null} 1501161387089 Sync.BrowserIDManager ERROR Could not authenticate: {"error":{},"errorString":"[Exception... \"NS_ERROR_CONNECTION_REFUSED\" nsresult: \"0x804b000d (NS_ERROR_CONNECTION_REFUSED)\" location: \"JS frame :: resource://services-common/rest.js :: onStopRequest :: line 483\" data: no]","message":null,"code":null,"errno":null} 1501161387089 browserwindow.syncui DEBUG observed: weave:service:login:error 1501161387090 browserwindow.syncui DEBUG onLoginError: login=error.login.reason.network, sync=success.sync 1501161387091 Sync.SyncScheduler DEBUG Clearing sync triggers and the global score. 1501161387091 Sync.SyncScheduler DEBUG Next sync in 3600000 ms. 1501161387092 Sync.ErrorHandler ERROR Sync encountered a login error 1501161387094 Sync.Service DEBUG Exception calling WrappedLock: {"error":{},"errorString":"[Exception... \"NS_ERROR_CONNECTION_REFUSED\" nsresult: \"0x804b000d (NS_ERROR_CONNECTION_REFUSED)\" location: \"JS frame :: resource://services-common/rest.js :: onStopRequest :: line 483\" data: no]","message":null,"code":null,"errno":null} 1501161387094 Sync.Service DEBUG Not syncing: login returned false. 1501161387109 Sync.ErrorHandler DEBUG Addons installed: 0 1501161387109 browserwindow.syncui DEBUG loginFailed has sync state=error.login.reason.network

Delete the signedInUser.json file in the Firefox profile folder and delete the Weave folder (Sync was formerly named Weave).

Remove an existing chrome://FirefoxAccounts entry in the Firefox Password Manager. If that isn't enough then delete the prefs.js files to reset all prefs and maybe also delete the compatibility.ini file to reinitialize the profile.

Delete and re-create the Firefox account or change the Sync password. Note that changing the Sync password will wipe all data stored on the Sync server.

Can you log in to the Sync account via this page?

• https://accounts.firefox.com/settings

You can use the button on the "Help -> Troubleshooting Information" (about:support) page to go to the current Firefox profile folder or use the about:profiles page.

• Help -> Troubleshooting Information -> Profile Directory:
  Windows: Show Folder; Linux: Open Directory; Mac: Show in Finder
• http://kb.mozillazine.org/Profile_folder_-_Firefox

Thank you. Managed to get Sync working again after Deleting and re-creating the account again. However, strangely, Sync did not set up correctly until I added a 3rd device which wasn't a desktop.

It only worked after Firefox was installed on an Android Phone and after the recognition of that device the sync started to work. Strangely, the Sync is now working on both the desktops but not on the phone :-) Thank you all for the help.