I think I would first try rebuilding the prefs. You can use a non-destructive version by referring to Jacobs post here: https://forums.adobe.com/message/6461768#6461768
After that it might also be a good idea to try and copy the contents of the old file to a newly created one to rule out anything that might have happened to the file.
And of course: don't just save, but save as in order to not save an old version.