Hello again... Back from dentist after 3,5h
Ok, back to topic...
@Davis:
That's all clear at this point but if I create a user in Calibre how can this user login from the minimalist Calibre server WEB-GUI? There isn't any menu or anything else that a user can login with the former created credentials ...
And yes... I have former think that the Calibre on Windows i.e. is usable as an Frontend to a running Calibre-Server-Instance. And that I crash the DB if I use the same folder from different installations is clear like water...
@chaley:
Please ignore the typo "caliber" instead of "calibre". That is my auto-correction I use in Windows that don't know calibre before

And yes. The given path point physically absolut to a share on a RAID10 cluster on the same Debian machine where the Calibre Server is running. This SMB-share is shown in LAN as "UNC:\\SV-OMV-NAS\Dokumente\Calibre" .
Greetz
Micha
(need more IBU...)