Start Thunderbird in Troubleshoot Mode. The following article explains how to do that and why it can be useful to help identify the cause of a problem:

https://support.mozilla.org/kb/troubleshoot-mode-thunderbird

What's your operating system? If Windows, start in Safe Mode with Networking mode:

https://support.microsoft.com/help/12376/windows-10-start-your-pc-in-safe-mode

Does any of that cause the problem to go away?

Or - since the Thunderbird team is apparently unable to follow your steps and fix this mess - I can just use Evolution and stop my donations. Hmmm...

Thanks for making the choice easier.

Or - maybe reply with information when someone attempts to assist you? https://support.mozilla.org/en-US/questions/1486505

Closing - duplicate of previous question.