There might be an issue with profile folder. I would try a back-up for what is now on profile folders and after that downgrade to v68 version to see if there it's working.

Thanks very much for your quick response. I have downgraded to 78.0 and everything went back to normal!

You mean you downgraded to 78.2.0 ? (not 78.0)

Well, it says 78.0 => https://ftp.mozilla.org/pub/thunderbird/releases/78.0/win32/nl/

Well, I have been too optimistic. After restarting my PC, Thunderbird didn't work properly anymore. It started asking for my password an infinite number of times, even though I filled it in correctly. No messages could be retrieved or sent anymore.

I tried to remove the account settings and create a new email account. After filling in my name, email, password and all the necessary server information for Gmail, and after allowing Thunderbird to function as a client for my Gmail account, the settings seem to be correct and show a green checkmark. But then, when I click "Done", I get an error message: "Incoming server already exists". I tried many times...

I cannot find any useful information online. How to solve this?

I tried to remove the account settings and create a new email account. After filling in my name, email, password and all the necessary server information for Gmail, and after allowing Thunderbird to function as a client for my Gmail account, the settings seem to be correct and show a green checkmark. But then, when I click "Done", I get an error message: "Incoming server already exists". I tried many times...

wvthor, were you able to resolve your problem? If so, how?

Hello Wayne, Well, I did what svlad2009 suggested and downgraded to version 68. Then I got things working again. Recently I upgraded to version 78.4.3, and that went fine. So it seems to be resolved. No idea what the actual problem was.