It might just be that the suffix of the file (.com) needs changing to .mov or .m4v. Try this first. The fact that the error is talking about a song, not a video, makes me think that it hasn't sussed out the file format.
I've found that the iPod is quite fussy about the format; I'm using MPEG Streamclip for encoding (which is absolutely excellent), but about 1 in 5 videos isn't accepted by the iPod (it could be that I've not got the encoding settings quite right). I can fix these by loading them into QuickTime Pro and then re-exporting them.
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