Kmail badly broken

Hi!

I'm getting a bad error from KMail:

"Im E-Mail-Programm ist ein schwerwiegender Fehler aufgetreten. Das Programm
wird beendet. Die Fehlermeldung lautet: Could not create collection trash,
resourceId: 2"

(Translation: "In the email program, a grave error occured. The program is now
being terminated. The error message is: Could not create collection trash,
resourceId: 2"

Before that, I've done something with trash folders (local one and in my local
Dovecot IMAP server).

The KMail version is 5.7.3.

Any ideas?

If I was to set up and configure Kmail again, which files in ~/.config,
~/.local or ~/.kde (or others) should I delete?

Bye
Volker