Hi Icosahedron
That happens to me one time without apparent reason, I check all they variable, interface, software, Even memory available in they qy, but everything seems ok, it was just they qy didnt play the midi song. The problems was a Lot of midi messages in automatization between notes, I set modulation for midi velocities en several tracks and was to much for the little qy, I cut the number of messages and problema solved.
I hope that helps.
Cheers.