Please change your password NOW, as it appears in your screenshot. Do it now for your security.

The screenshots are not showing for me. Please post again. Thank you.

here are the screenshots

Try the entire process again. That is, remove the account and start over and this time OMIT entering the password. After clicking DONE, google will prompt for it.

Thanks David, I tried, but google did not prompt for it (see screenshots)

I duplicated exactly what you did, and received the google prompt. You showed two screenshots. For the one with the message that it could not log in, did you press DONE as I stated? When I did, the prompt appeared.

yes, I pressed the blue botton (see screenshot), after that came to error message "Unable to login..."

I understand what you mean. Just as a test, I deleted "info@kanai-zimmermann.ch" and created it again without the password, and the accounted was automatically installed...

I also just deinstalled and reinstalled thunderbird again, but it did not help either.

Really a mistery to me....

Do you have any other idea?

Are you using an antivirus like AVG/Avast, or any other non-MS antivirus, or a VPN?

No, I don't

Wow, it worked, I found the problem. I cleaned out all the old passwords on thunderbird, and one of them was for info@ivo-zimmermann.ch. After that, it asked for the Google Login and then it was done. Thank you for anyway for your efforts!

That's always the issue; Google wants no passwords at setup. Thanks for verifying it.