It is important to use Developer Utilities to rename solution files. They ALL must be opened stand-alone and same time even if you only want to rename a single file. Otherwise you can break your file references.
Here is a recent post which explains it:
Might it be this issue? BTW, I didn't catch all the places it could break; I'll bet most of you will be surprised by at least one of the ways that manually renaming files can break things, as well. :-)
BTW, if you immediately rename the files back to their original names, you will be okay. But if you've changed anything such as corrected a broken import or re-pointed the file reference to your DEV, then you have stepped the creek. Best to grab the backup and start over - renaming the files using DU first. Second-best option is to then manually undo your corrections then rename the files back to their original and step forward properly.
ADDED: Another issue is that FMGo is case sensitive on file names, there should be no periods other than the extension period.
Thanks for the hint, LaRetta. I didn't know Developer Utilities existed.
It seems to have mostly done the trick. I haven't had the mystery reference crop up since I renamed the files in that manner.