I recreated the Prism 'application' and now it works fine.
Thank you for all your help and answers. Finally I subscribed to Pro and have already enjoyed several self-created matches.
However I am quite disappointed that in my region (Middle-Europe) how small number of players play premium content. I must agree that at least some of the premium maps (those which were free in the past eg. Skyward or Aerowalk, not necessarily those created especially for QL) should be opened to public.