I’ve now setup a new mailcow docker server, but with the use of the volumes. The original problem doesn’t exist anymore.
But after accessing my mail folder with thunderbird all the old mails are gone. Only new ones are displayed. The mailcow admin gui shows also, that the mailbox is empty. But if I’m looking at the vmail-vol-1 volume, I can still see them (and also decrypt them with dovecot).
I already ran the following commands without success:
docker compose exec dovecot-mailcow doveadm force-resync -u user@mydomain'*'
docker-compose exec dovecot-mailcow doveadm quota recalc -A