Plextor PX-708a cannot write Mount Rainier on DVD+RW

I got this drive today and at first I was really satisfied. But as I treid tu burn a DVD+RW in MRW format InCD refused and used InCDfs instead. The Plextor Site says that the drive shpuld be able to burn MRW on DVD+RW.
InCD, Nero and the firmware of the drive are the newest versions.

Has anybody tried this before? Does anybody have the same problen or knows how to fix it?