sorry for beeing late.
The problem was solved about a year ago. I do not work for this customer any more because the work is done. And I do not have access to the code any more.
What did I do? Hmmmmm
If I remember right then I 'opened up' the rights in the call, which means I allowed everyone everything on this mailslot. Please look at the description of the function call.
With this new string everything worked. And then I added carefully one restriction after the other and finally we had the restrictions as desired.
So the way was: do not experiment with possible restrictions, simnply allow everything (and for everyone). Test this. Then carefully add one restriction and test this. And so on.
Sorry, I could post the exact code that worked.