I can't give you any real input about this problem, as i'm not using an 68030 currently, but,
Then I re-installed MUI and ONLY MCC The Bar.
Version 21.4 is ok. Version 21.5 does again the System Error $8000000B.
Imho, someone must convince the developer of YAM to stop forcing users to upgrade their MUI classes to the latest available versions. Some may be buggy as itix pointed out, while others are bloated with myriads of useless, added features. I think that the recent versions of TextEditor.mcc is a good example of this.
