This is the error message in English: "We could not sign you in. Try again from a different device."

Hier ein ähnlicher Fall / Looks like this is a similar issues: https://www.thunderbird-mail.de/forum/thread/97589-einrichten-von-yahoo-mail-konto-unm%C3%B6glich-etwas-ist-schiefgelaufen/?postID=563425&highlight=yahoo#post563425

Hello there

Give that browser access to use Javascript. Using heavy antivirus can control these log in processes . Turning of these antivirus and look for a possible clue , try to use it now and log in. This is how to check for the browser. Try to update all involved parties. Did you see all settings for the Smtp ,they where all oke?

I've checked SMTP again: smtp.mail.yahoo.com, Port 465, normal PW, SSL/ TLS. Javascript: enabled.

I've tested App_PW as well.

The answer is: "We could not sign you in. Try again from a different device."

When I copy the log-in-link to Chrome, I can proceed the process and agree to the access of Thunderbird to my yahoo account. But after that there is another error-message. I can add some screenshots later.

Hello there

Yes okay , show me all these errors of that or these issue s , are you sure that you do have more issues? https://support.mozilla.org/en-US/kb/how-to-make-screenshots Can you use recovery mode for these accounts?