People... I feel really embaresed now!
I got really panicked, because I knew a problem like that can generally mean a sound board issue - the beginning of the end for that keyboard...
I tryed everything, factory restoring, all the possible sound settings, etc.
Then I got the ideea to listen through headphones, and sudenlly the problem was gone. I thought it had something to do with the main outs, tried the other outports.... then it sudenlly hit me: the mic was on. It was feedback!
Something in the acomp frequencies triggered it. Turned mic off, everything back to normal. That was all!
I know...embaressing. Had a good laugh after.
Sorry. Thank you all for your implication!
Moral: sometimes things are so simple that we can't help looking at them from a complicated perspective!
Should I delete the thread?