Itunes could not copy because the file could not be converted

Problem Fixed:

Remove the song from your itunes library, then go to the itunes store and try to download it again. It will say something like "this song has already been purchased, would you like to download it again for free?". Click yes, and it will download again. This worked for me, hope it helped

This may work for songs obtained through the iTunes store (although you're going to lose the metadata you stored with them, like your ratings, custom artwork, etc.), but it doesn't translate to music you added from elsewhere (I know: I've tried).