I think I know the problem:
I used Wayfarer before and I recognized that some progs, started after, will crash unforeseeable, or show a strange behaviour (e.g. Mplayer, OWB, ImageFX) - not allways but often - not really reproduceable - that makes it not easy to find out what happens, because system freezes always complete - only coldreboot helps..

After turning of G5 completely off (needs to be disconneted from powersource), Morphever now does what it should do.
Seems that Wayfarer leaves something disruptive in RAM, after quitting.