I had to set up a stratum server to get the full hashrate (thanks to slush for his work!).
A big THANK YOU! to friedcat and all the other people involved in this, also for providing the User Tips. Didn't have that document before.
One question remains: What's the clock switching for? If I switch the clock to "high", the power consumption increases by about 15W, but the board stops working (hashrate displayed on the pool website goes down). After 2-3 minutes, it resets itself and starts up with low clock. If I save the high clock settings, it starts up with high clock, doesn't work and restarts after a few minutes.
Strange that the other guys claim 13GH without overclocking. Is this because of differences in calculating the hashrate or other reasons?