After the manual trigger of the initial srub as proposed by @joeschmuck the Scrub Task seem to work now.
I have created a bug ticket regarding this issue:
https://ixsystems.atlassian.net/browse/NAS-132007
Hint:
There is another thread having the same issue:
https://forums.truenas.com/t/scrub-tasks-not-working-for-one-pool/21615