Please turn off Thunderbird and restart it in safe mode: https://support.mozilla.org/en-US/kb/safe-mode-thunderbird

This is a great begging. Thank you on your fast reply

Is it permanently gone, i.e. if you restart again normally you no longer see the problem?

Wayne Mery said

Is it permanently gone, i.e. if you restart again normally you no longer see the problem?

Unfortunately my problem is not permanently solved. The moment I turn to normal mode problem is still there. It appears that there is a conflict with the 2/3 add ons that I use. Both have to do with security and encryption. When I turn them off and restart everything works.

> Both have to do with security and encryption

could you be more clear, with names?

(There are persons interested in details)

Sure, I am using:

1. Secure Addressing 1.6

2. Encrypt if possible 1.5

There are a number of add-ons which have caused this, in my case Quote & compose manager. If you are using that, this is the update link: https://freeshell.de/~kaosmos/index-en.html The issue is fixed for me. :-)

Yes several different add-ons can cause this conflict. In my case, I still have problem with Secure Addressing 1.6 if activated. Some add-ons have applied updates to fix this problem and work fine and some not. My case is valid also with v45.1.0