Sorry that I forgot to mention: The manual backup always works. And during last night's backup something strange happened: The problematic courses got saved, another one instead failed:
01:50:17 Start backup course GK 11/II
01:50:17 Phase 1: Checking and counting:
01:50:17 checking parameters
01:50:17 calculating backup name
01:50:17 calculating modules data
01:50:18 calculating users
01:50:18 calculating logs
01:50:18 calculating user files
(it stopped here)
I am quite sure the nothing has been changed in the those courses since the second to laste backup (we have autumn holidays in Germany and the lapse of time between the two latter ones was only a few hours at night).
If it is of any use, here is the backup summary of my courses (»Fehler« means error):
What else could I do?
Thank you very much again