So simple it can be .. facepalm. Thanks for testing and your quick reply!

Starting emulation with -device sm501 seems really to fix it.

I would have not expected such a bug having such a small impact to the normal system. It only seems to affect BltBitmap(), which seems not to be used much if at all in normal MorphOS code (understandable). But it is used for rendering old style gadgets, not even the text of the gadgets (radio buttons, listviews..) gets blitted correctly..

I'll report the bug.