What version of Thunderbird? What is your account Type - POP or IMAP? What is your anti-virus software?

Please respond to your support forum topic.

Version 128.2.0esr IMAP Panda Dome

You most likely experience the folder corruption problem described in this bug. https://bugzilla.mozilla.org/show_bug.cgi?id=1890230

Try to repair the affected folder(s). Right-click the folder - Properties - Repair Folder

Wrt anti-virus software, it is recommended to create an exception in your anti-virus software for the Thunderbird profile folder, so that the anti-virus real-time scanner will not scan it. https://support.mozilla.org/en-US/kb/profiles-where-thunderbird-stores-user-data#w_how-to-find-your-profile

Ich bin leider nur Anwender und kein Programmierer. Daher verstehe ich die vorgeschlagenen Lösungen nicht. Mir würde nur eine Schritt-für-Schritt-Anleitung weiterhelfen. Trotzdem danke ich für den Beitrag.

As already described in the previous post, it's two simple steps:

Try to repair the affected folder(s). Right-click the folder - Properties - Repair Folder

Can't be that hard.