First of all, a big thank you to all the people involved.
Probably there was indeed a problem with the storage change for the database. However, the interesting thing is that this setting was done before the first account was created. So it should be safe to assume that the “new” setting will be applied when the accounts are set up. Apparently, however, there is an error behavior here.
I will now apply the hint that I first set up the mailboxes step by step and observe the behavior. If it happens again, my decision will quickly go in the direction of Microsoft Outlook. There are not such fatal errors of this kind.
Once again, thank you very much and for me the topic is closed for now. @all: Have a nice weekend!