Does it also happen if you do Help > Troubleshoot mode ?

Unfortunately yes, using troubleshoot mode, disable all add-ons + customizations, restart in safe-mode, same issue... also the sender´s mailaddress and my mail-address in this mail is empty...

I understand the frustration you're facing with the empty emails in Thunderbird. This issue might be related to a compatibility problem with the new version. To troubleshoot, you can try a couple of steps. First, make sure your Thunderbird is up to date. Then, I'd suggest creating a new Thunderbird profile and see if the issue persists. If that doesn't work, you can consider checking for any updates for your add-ons or themes. Sometimes, conflicts can cause such problems. Lastly, if none of these solutions work, let me know.

Thanks for the hints, but let us close this topic!

I did not have the issue again and I can rember I had during the same time somehow a message like "could not move the mail to the sent folder" or similar. I am not sure why this was the case and if this message is related to the "empty mail" behavior or if this was just a good timing.

Anyway, looks like this is just a single case and the mail is still existing on the server with content, this is sufficient. I don´t want to block you with this, you probably have more important stuff to solve.

Thanks again!