Sorry, even as a long term user I didn’t expect to have openHABian logging for the amanda setup, but yes - I should’ve known better! 
Yes, I use CIFS, because I use it in my whole environment, and as “slots” were created…
But then, as I read the openHABian logs, I stumbled across the “symlink”-creation problem.
So, I changed my /etc/fstab and added mfsymlinks to the mount and now it worked:
//192.168.XX.YY/Backup /mnt/synology/ cifs dir_mode=0777,file_mode=0777,username=USER,password=PASS,mfsymlinks 0 0
for someone searching for it, here’s the log output for reference:
[...]
$ chown --recursive backup:backup /mnt/synology/openHAB/slots
+ chown --recursive backup:backup /mnt/synology/openHAB/slots
+ return 0
+ cond_redirect ln -sf /mnt/synology/openHAB/slots /mnt/synology/openHAB/slots/drive0
+ [[ -n '' ]]
+ echo -e '\n\033[90;01m$ ln -sf /mnt/synology/openHAB/slots /mnt/synology/openHAB/slots/drive0 \033[39;49;00m'
$ ln -sf /mnt/synology/openHAB/slots /mnt/synology/openHAB/slots/drive0
+ ln -sf /mnt/synology/openHAB/slots /mnt/synology/openHAB/slots/drive0
ln: failed to create symbolic link '/mnt/synology/openHAB/slots/drive0': Operation not supported
+ return 1
+ echo 'FAILED (link drive0)'
FAILED (link drive0)
+ return 1
+ return 1
+ '[' 1 -ne 0 ']'
+ whiptail --msgbox 'There was an error or interruption during the execution of:\n "50 | Backup/Restore"\n\nPlease try again. If the error persists, please read /opt/openhabian/docs/openhabian-DEBUG.md or https://github.com/openhab/openhabian/blob/main/docs/openhabian-DEBUG.md how to proceed.' 14 80
+ return 0
+ true
[...]