With the “alternative rsync” (see my older article on rsync and Synology!) things started looking a little more successful.
Apparently a lot of files by the name “@eaDir” get created “automagically” in various subdirectories. And as I usually start rsync with “–delete-after”, they all get removed in the end. I am not sure, whether the DiskStation OS (“DSM”) likes that, so for the time being I will do w/o that command line option. I will investigate the “@eaDir” issue “in the near future” (TBD).
Another issue: lots of messages like “cannot chgrp …”. Apparently that’s because the group identifier on the source system does not exist on the target system. I definitely don’t want to see that message, so something must be done in that area (TBD).
It really is a relief to have a regular backup procedure in place again.