After I found out whats wrong with the crash I changed the morphunits on my harddisk to open/close library in initialization and finalization that I can continue testing the behaviour.
I removed some hacks I introduced for this problem and all work very nice. The next step was to activate my own MUI Class again and see if the paint event work as expected. It seems to work but the colors does not look like expected. The reason seems to be that my Buffer rastport only has 8 bitplanes, hence direct color setting does not work. I found our that the window rastport tells me that it only has 8 bits (even it should have 24 as the Ambient screen has 24 bits) So when I create my buffer RastPort and Bitmap I should not use the Depth from Rastport and also not set the Window rastport Bitmap as friend of my bitmap or it end up with 8 bits and the PenMode can not be disabled.
But finally I got it to run and to show that it work I compiled my old (AROS) PaintBox example and the little ColorIt Game.
The fonts are much smaller than on AROS which is little bit strange, and the comboboxes are still missing (see MUI is picky) this I will include later.