OK guys, problem solved. BIAB seems to overreact to my expression pedal (Boss EV-30). It seems BIAB received (or at least thought it recieved) an endless stream of low expression values and sent them on to the midi device. When i un-hooked the pedal it sent an expression 0 (!) but when i started BIAB without the pedal connected from the start everything suddenly worked as it should.

I then tested the pedal i Cubase and it worked as it should. No endless stream of midi messages from the pedal and the Yamaha plays as it should + the pedal works like an expression pedal is supposed to work.

At least the combination Komplete Kontrol and Boss EV-30 doesn't seem to be compatible with BIAB.