I was just looking at someone's solution and upon opening one of the files it said the data source could not be found. Dialog popped up to look for it and I would point it to the correct file (which is in the same folder as the one looking for it). File opened as normal and manage database was fine. Then I'd close the file and reopen and it would ask again. I did the same thing. Then went to manage data sources and checked the file that was referenced there - it was correct - file:filename.fp7. No need for directories etc as it's a relative path. So I edited the data source, deleting what was there and browsed to the location of the file and selected it. Clicked ok etc and closed the file. Open the file and it again asks for the data source for that one file.
I'm thinking the file might be corrupt (probably the one I'm opening that is looking for the other file), but wondering if anyone else has come across this or any other things to try.