I had similar problems with my X4... I'm not 100% sure, but I think it might have to do with the fact that the midi file has more than 16 tracks. You can do a quick test: edit one of these "song error" midi files in a software sequencer and remove enough tracks so you keep only the first 16. Save and check again if it works...

Be careful with file names with over 8 characters, at least it seem to be a problem in my X4...

Good luck,

-- José.