Do you have an anti virus? Their scanning of encrypted connections has been known to cause this error.

Yes, Norton 360. I turned that off & it didn't change anything. Still had the problem. I also had tried changiing the Thunderbird TLS security from 3 to 1. That did nothing either.

I suggest you change the TLS back. That was never going to be in any way helpful as you clearly are communicating with the server, just that some network traffic is being messed up. Honestly I think by Norton.

I may be particularly biased, as it is now more than 20 years since my son dubbed Nortons as "Norton's resouce stealer" but I never help anyone around here (friends and family) when they have a Norton product that I do not end up having to disable half the Norton's product features before it allows Thunderbird to configure a new account and function correctly.

Commonly I find I need to disable something called sonar, as well as mail scanning and phishing protection. Try restarting your system in safe mode with networking and see if your network issues continue. BTW. Norton will disable itself it says. But somehow that is not what happens. Only operating system safe mode (where you still have Microsoft defender) or uninstalling it can allow the system to function without it interfering.

To restart in safe mode with Networking. Which loads only the very basics needed to start your computer while enabling an Internet connection follow the instructions here

Hi Bradford, I have this issue as well at around the same date, after a recent Thunderbird update. I had the same version of Eset Internet Security before and after the update.

I removed the program and reinstalled it from an old install file, and it went back to working properly.

However, I had left it to update automatically so I am now back where I started. Hoping someone has a solution, or I will have to drop Thunderbird after a decade or more, as I need those sent email records on the server.

Also, check this post out. It tells you how to get an older version. I had my old file in Downloads but deleted it. https://support.mozilla.org/bm/questions/1347599#answer-1436206

I note the commnets about new security updates on this update for released 16 Sept, that "that shipped in Thunderbird 102.2.1" I wonder if that had something to do with the issue? https://www.thunderbird.net/en-US/thunderbird/91.13.1/releasenotes/

I note 102.2.1 that you are running I think, was rolled out 1 Sept which seems to align with both our issues around accessing the Sent folder on the server.

One of my two email accounts in Thunderbird stopped saving Sent copies on 9 October, just after an update. (Luckily, I automatically bcc myself in every sent message, so I have copies in my Inbox.)

I have just solved it in Account Settings - Copies & Folders - "When sending messages, automatically: Place a copy in:" by changing "Other: Sent on [my account]" to "Sent folder on: [my account]"

See screenshots.

Sounded like the same thing, so it was a very annoying bug.

(Account Settings are quite hard to find: stuck up in the very top right corner when you view your account folder.)

Yes, this just recently started happening to me also on my debian 10 exim4 accounts hosted on my own server (my yahoo account still seems to file a copy in the 'sent' folder just fine); it was apparently working fine as of a ~ couple of months ago, but apparently auto-update (currently v102.5.0), WHICH I DON"T RECALL EXPLICITLY ENABLING TO BEGIN WITH!, may have likely horked something (?). I'm not aware of having changed anything of significance on my server in that time frame.

At around the same time, ThunderBird started complaining that it no longer considered my security certificate to be valid (apparently not recognizing that my 'wildcard' domain should match!? it sure looks valid to me, with a date extending into next year), but then does allow mail to be sent if I 'confirm a security exception'.

I have tried suggested 'TLS security' level lowering and 'connection security' type changes without fixing it yet.

WTF?

[Edit: AH! seem to have found a workaround. If instead of choosing 'Get Messages' with the account selected, one chooses it with the 'Inbox' selected, yet another 'Security Exception' dialog indicating ~ that it can't figure out how to match my wildcard domain pops up; if I agree to the exception there yet again, sent messages are subsequently filed in 'Sent' again!]