I know you've solved the issue by re-ripping, but I have found an alternate workaround to the dreaded "cannot be played on this iPod" error. I'm posting it because 1) I have encountered the error numerous times and it takes me awhile to remember the fix and 2) I searched the discussion boards and did not find anyone posting this particular method.
I've encountered numerous files (most of which I ripped from CDs YEARS ago) that will play in iTunes but not copy to an iPod, with the "cannot be played" error. Attempting to convert them to MP3 within iTunes results in an "error -50" which is of zero help. What I discovered is that when I look at the files in the Finder, they are invariably listed as "MPEG" not "MP3". Correcting them is simple, but it can be a tedious one-at-a-time process if you have a lot of them. The process is as follows:
1. Highlight the song in iTunes.
2. Right-click (ctrl-click) on the song and select "Show in Finder".
3. Assuming the file kind is "MPEG" or something other than "MP3", edit the filename and add ".mp3" to the end of it.
4. Go back into iTunes and PLAY THE SONG. This is important, if you don't play the song, iTunes apparently doesn't recognize the change. You don't need to play the entire song, just a few seconds will do.
5. Sync your iPod. The songs should now copy properly.
Hope this helps!
