When renaming very long files, or very long directories, mp3tag sometimes reports a "file not found error" i can usually bypass this error by first moving the file to "higher" directory i.e. root and then renaming.
This may have to do with a filename/folderpath limit in Windows. I don't know the exact limit but I think when the total filename (including path) is longer than 255 characters (2**8 - 1), then it can't handle the files.
I got a similar error for some medleys (containing seven or eight long song titles) with a title length of over 100 chars. With everything from folders and other info, the filename made problems. But I'm not sure about this, as I'm not an expert when it comes to the filename limitation of Windows.