Outlook Exchange no connection - DB issues

Dear Community,
I have tried to setup a new domain and wanted to create a first mail address to use this with Outlook via Exchange.
Unfortunately, I get always the message disconnected on Outlook.
Within the Logs I could find the following:
Gromox http:
sqlite3_exec(/var/lib/gromox/user/mail.de/max-mueller/tables.sqlite3) "INSERT INTO t56 (folder_id) VALUES (1900545)": NOT NULL constraint failed: t56.depth (19)

Admin API:
[WARNING] (security) getUser() failed (OperationalError): (MySQLdb._exceptions.OperationalError) (2006, 'MySQL server has gone away')

I'm using the appliance and the DB seems to run.

Where can I check the credentials? Any other ideas?

yes I did. I also found out that the autodiscover seems not work. I just don't know why it is not kicking in with this domain. The other is working

probably. I'm just wondering why one of the two domains is working and the other is not working...

I have checked again my DNS Settings.

From my perspective it looks ok. Any thoughts?
The Windows Desktop is also able to ping anything incl. nslookup.

Any idea?