Updating the program will not do this. Something else has happened to cause this. I suspect file errors, hard drive errors, and/or corruption of the Thunderbird Profile. When an account in Thunderbird is created, it is created as either a POP or an IMAP account. Afterwards, it is not possible to convert an account between those types. A new account has to be created for the other type. Just changing the server name for the account afterward will not change the type of account (POP or IMAP). You say<blockquote>Forwarding and POP/IMAP are still properly set</blockquote>which is confusing. I don't know what you mean by POP/IMAP being still properly set, when you say the type of account (POP or IMAP), has been changed (which I have said, is not possible). '''You have two options for what to do now.''' The first is to completely ignore the cause, and just deal with the effect, leaving the possibility of more things going wrong. The second option is to deal with the probable causes by doing some computer maintenance and create a new Thunderbird Profile. '''Option 1''' Create a new account in Thunderbird using that e-mail address, using POP. Then move all the e-mails from the IMAP account to the POP account. '''Option 2''' '''1.''' Immediately [https://mzl.la/2lBfidM#w_backing-up-a-profile backup the Thunderbird Profile folder]. '''2.''' Backup your whole computer. '''3.''' Run a thorough Check Disk. [http://brucejohnson.ca/CheckDisk.html How to run a "Check Disk" in Windows] '''4.''' Run a Disk Cleanup. [http://brucejohnson.ca/DiskCleanup.html How to run a "Disk Cleanup" in Windows] '''5.''' [http://brucejohnson.ca/ThunderbirdProfileNew.html Create a new profile in Thunderbird], then [http://brucejohnson.ca/ThunderbirdDataImport.html import your stuff from the old profile to the new profile.]

Thank you. The laptop has been overheating and shutting down, so you are right on. I am glad to hear that the update was not the issue! Thank you, also for the fix suggestions. FranouF

A good start is get your laptop thoroughly cleaned. Also, perhaps some application is misbehaving, sucking too much CPU. Check task manager, process tab