I ran it with the lowercase-x-option. That does not seem to cure it. However, running with the uppercase-C-option and lowercase-x-option - that combination finally worked.
In my opinion, the uppercase-C-option cleans-up the temp-files at runtime once the application terminates. Shall this be reported as a 'bug' (in this case I would not know who to contact) or should it be silently accepted? What is your advise?