Thanks rharv, that solves the problem! In M-Audio FTU, the "Input Monitor" realtime effect sent from RealBand/PT always goes to the same output pair as the input (i.e. output 1+2 for input 1+2). If I however assign all audio tracks to a different output channel, (e.g. output 3+4), the problem disappears.

Thank you all for your help!

Regards
Bjorn