I tried to SnoopDos the system, but didn't get a clue.
SnoopDos produces the same log as in the 4000s. All the libraries that the MUI prefs program tries to load are there, except powerpc.library, cybergraphics (I don't have neither PPC nor gfx card) openurl and screennotify.library. The same libraries are also missing from the 4000s, but no problem there.
Then I re-installed MUI and ONLY MCC The Bar.
Version 21.4 is ok. Version 21.5 does again the System Error $8000000B. Now I really suspect there is something wrong with the latest versions of YAM and the 3 MCC. Is there anyone using YAM 2.5 on a 68030 with a similar config as me (OS 3.9, no gfx card, 32M fast)??