Repetier Host for Mac crashes repeatingly (no replies)

We are running a single extruder Creatr on a Macbook, but Repetier-Host crashes continously (about 20 seconds after the printing started). Setup was fine, STL loading is fine, Slicing is fine, heating the machine is fine, just about 20 seconds after the printing starts, Repetier-Host crashes and printing stops.

System:
- Creatr 3d printer
- Macbook Core2Duo
- Mac OS X 10.7.5 Lion
- Repetier- Host 0.56

Any idea's what could be the problem?

Andreas