Thanks to wahlerstudios for the help. Here's his reply from the forum:
Usually this has to do with mono and stereo tracks. In a configuration of 34 tracks, only one stereo track is allowed, which is track 33+34. Unlink any stereo track in the 1-32 range. It also can help to unlink tracks 33+34 and link them again.
That's exactly what the problem was and his solution worked.