Thanks Eloy for your help, your analysis was 100 % right!
If there are symbolic links to include other directories and such a link is "recursive" this will produce a moodle "backup-loop" until disc is full. This will only happens if in the backup configs "include coursefiles" is enabled.
Maybe Eloy may find a solution to stop such a LOOP and a possibility of a special ERROR - E-Mail for failed backups
I am happy, that my discs dont run out of limit again ....