Philip, I strongly suggest you try other pools like mining pool hub or coinotron, these last two seem to be much more reliable IMHO.
Thank you all for your kind replies. I think there are two possibilities to explain suprnova behavior:
1. PPLNS number of shares is lower than a few unlucky rounds, therefore the last contributors to a round get overpaid and the first ones get stolen.
2. The admin is deliberately playing around.