I installed the current duplicati package from the Asustor App Center. That worked fine.
But then I wanted to get the latest Canary (because of many fixes).
Changing the update channel and checking for updates did not work out.
Cyril Pawelko provided the right solution here:
It seems, that this needs to done again after a reboot of the NAS.
Hope this helps other folks!
Can someone mark this as solved?