For just syncing my 2 star and above music it was working fine. If I added my un-rated music I'd get the above error. To fix, as everyone says - I would unsync all the music, and resync my 2-star and above music. But if I added the extra "unrated" folder it went bad again.
So there seems to be an error in the database, fixed by unsyncing those items and then resyncing.
See:
http://support.apple.com/kb/TS2830 for that minor error.
But there is another error caused by a problem files, which won't be fixed by just resyncing.
In summary, after reading through this thread, these things help find the culprit item:
. minor errors - just unsync music, resync music
. remove song with exclamation mark (on left of title)
. remove song with exclamation mark in the name
. rename voice memos back to the original names (no comments etc!)
... or just try removing voice memos
... and unclick "sync voicememos"
. I had a pdf file in a playlist
. I had to UNclick "sync music videos" - not sure what was wrong in my video (yet)
. out of date podcasts
. someone mentioned their problem was in the PHOTOS, not music
Hope that helps someone
edit: Resynced music videos (I only have 3). It shows successful sync, but I can't find the music videos in the iPhone anywhere.