Thanks for your answers.
After i read them i would like to check your ideas. The Keyboard was off for maybe 1 or 1,5 hours, i turned it on, now with headphones inside, and all works fine. Hhmmm, ok i played for a while and after an hour the sound goes off completely. I took the headphones and that sound was ok. After 10 seconds I heard a croaking sound through the headphones and was a bit confused. I took them off and heard the sound coming out of the speakers again. After a few seconds the sound returned to normal. For the next half hour everything was fine. I didn't do anything while this was happening. I didn`t play with the Master Volume or something else !
A thought about the cables. Both the left and right sides would have to be broken if the sound could fail completely, that's very unlikely, isn't it?
I still have my Ketron Audya connected to my mixer and it works without any problems. So I would rule out the mixer, power amplifier and speakers for now, right ?
I have to wait until the problem occurs again more often, like this morning. Maybe then I can try more.