Read in another topic on the same startpage for this forum, it did give me some information but not all, so the existence of this thread could still be useful.
Solved it like this:
[list=1]
[*]Added a new member to the Subscriptions table called ‘$aux_customerNumber’.
[*]Added ‘$aux_customerNumber’ to the safe attribute list (THIS WAS THE MISSING PART FROM THE OTHER THREAD)