I initially tried trashing preferences (Digital Performer prefs, MOTU Audio System prefs, MOTU 324 prefs. This will probably only help the first time.
You may have to reinstall DP or Audiodesk software. First, trash all of the following:

the DP folder
the FreeMIDI applications folder
The FreeMIDI folder in the System folder
The MOTU folder in the extensions folder
the 3 FreeMIDI extensions
Prefs for DP and FreeMIDI

***KEEP all of your DP Projects and your FreeMIDI configuration file(s)...
make sure you don't have any projects in the DP or Audiodesk folder when you trash it.