Hi everone,
I found the solution to this topic on a German speaking thunderbird forum.
For all non-german speaking people, here the translation with the action plan which solves the issue. At least for me.
Not sure though if all menu names are correct, as I only have German versions. But you get the point I assume.
Open ESET NOD32 Antivirus -> Setup-> Advanced setup -> Web and E-Mail -> SSL/TLS
Here disable the SSL/TLS-protocol filtering -> deakctivate
Open Thunderbird -> Tools -> Options -> Privacy &Security -> Certificates
Check if there still is an Eset certificate and in case it is remove it.
Restart the PC
Now without open any other programm or application go back to ESET NOD32 Antivirus -> Setup-> Advanced setup -> Web and E-Mail -> SSL/TLS
There you activate the protocol filerting again -> activate
Now open Thunderbird and go to ools -> Options -> Privacy &Security -> Certificates
Now check if there is an Eset certificate now. There should be one dated with current day and time.
Now Thunderbird worked as ususal again for me :)