An update, as promised...
Over and over again, this issue is reproducable.
So get rid of underscores and numbers in Application Roles' names and your life will be much more pleasant !
Thanks to everyone in my team (@bifacts, @bvdh80, Cindy and Marcel) for your valuable input in resolving this issue !
Things seem to go pretty well now.
It looks as if the underscores and numbers in the Application Roles' names cause these kinds of problems.
After having removed the numbers and underscores from the roles' names the error hasn't occurred anymore.
It's a pity that the documentation does not mention this restriction or naming convention.
Still testing though... I'll keep you informed.