So, with the kind help of roschmyr, I successfully got my MorphOS running again on my X5000.
That is the good news ;)

The bad one: YouTube videos do not work....

What could be the reason? I thought I had it already running with my X1650 before...

Are you sure, Jacek, you did not changed something?

cgxvideo.library is installed under MOSSYS/Libs.
I tried different sppofing options...

No idea. Please give me a hint, thanks!