Either the installer script didn't update the MUI libraries properly, or there is still an old version loaded in the memory or somewhere on your hard disk. Try rebooting your Amiga; if this doesn't help, copy the contents of the MUI directory from the YAM archive to 'MUI:libs/mui'.

Please also make sure you don't have a 'YAM:Libs' or 'YAM:mui/libs' directory in which old MUI classes are stored. The only path were #?.mcc and #?.mcp files belong is 'MUI:libs/mui'.