Solution for me:

The Error was probably that the Backup-Database was corrupt or not correct initialized.

I disabled "Local Backup" under "Open-E Gui" --> "Konfiguration" --> "NAS Einstellungen" --> "Einstellungen für lokales Backup".
Deleted the Logical Volume where the database was stored.
Created new Logical Volume for the backup-database.
I re-enabled "Local Backup" again.

Labeling of tapes works now.

Thanks for your reply.

Stephan.