I dont know why you have an export failure when you are trying to import. Are you sure you are doing it right?
TEST THE MUSIC FILES:
Copy some mp3s from your itunes folder into a new folder somewhere else on your hard drive. Try playing those same files in another media player like winamp, wmp or vlc. If the files work then the "files were not validly exported" error really doesn't makes sense because the files are fine. It must be an itunes issue.
Download the latest version and reinstall.
DELETE THE ITUNES LIBRARY:
but not the music. Maybe the library files are corrupt beyond repair. In that case back it up (just in case) then delete it. Try importing that test folder of music you made in my first suggestion to see if it will successfully add music.