Please post a screenshot of the error you get. https://support.mozilla.org/kb/how-do-i-create-screenshot-my-problem

There is nothing more but a window appearing:

"Das Zertifikat für imap.gmail.com stammt nicht von einer vertrauenswürdigen Quelle."

Seems to be a certificate issue - but out of nowhere. Was working the other day and I did not change anything - neither in TB nor in Google Mail.

I chatted with someone today, who regained function by deleting the account's password from Thunderbird.

Does that help?

Hi, well, at least it made me looking for the settings which I have not done in years. Seems that the authentification method was switched to TLS-certificate. I changed it for regular password and - well, it worked! Could now download my mails w/o any message appearing.

Thx a lot!