Correction: I forgot to undo snapshot.cfg before trying idea 1
I did that now and it works 
Unfortunately it’s not a solution for me as I need to access the whole file system of /dev/sdb1 an not only the subvolume DATEN.
At the end the solution was pretty simple:
I modified the /etc/fstab to first mount the subvolume DATEN to /mnt/SSD1TB_DATEN of the btrfs filesystem and in second line the whole file system to /srv/dev-disk-by-label-SSD1TB. It seems to be a BTRFS bug, as it will not work when I’m trying to mount the full file system first and in addition a subvolume of that file system to another path…
/etc/fstab:
/dev/sdb1 /mnt/SSD1TB_DATEN btrfs defaults,nofail,discard,ssd,subvol=DATEN 0 2 /dev/disk/by-label/SSD1TB /srv/dev-disk-by-label-SSD1TB btrfs defaults,nofail,discard,ssd 0 2
and I did switch the backup path from urback to /mnt/SSD1TB_DATEN
urbackupclientctl add-backupdir -x -f -d /mnt/SSD1TB_DATEN
Thanks uroni for your ideas and work on this!
For anyone, who has trouble with BTRFS subvolumes and Urbackup these different solutions may help