Here is what I learned from a Macromedia technician:
DW processes all .xml files in the Configuration/DocumentTypes folder,
so be sure to name your backup copy of MMDocumentTypes.xml to be
something like "MMDocumentTypes.bak" (as opposed to something like "Copy of MMDocumentTypes.xml").
I renamed the backup file I had created (Copy of ...) and the problems went away as soon as I restarted DW! Give it a try and good luck!