Hi SigiZ58,
Cubasis’ In-App Shop communicates with Apple to enable in-app purchases, which has no relations to the MySteinberg application registration process. Normally, buying in-app purchases within Cubasis and/or Cubasis LE should be a quick and easy process. As mentioned before, I’d think the cancellation of your app order seemed to have brought up the problem, and Apple should be able to tell what’s the problem here.
Best,
Lars