This is one of those errors that really don't have a clear explanation. Based on past history with it, either the HD of the Mac or iPod is going bad (this is in the iPhone section, so not iPod) or there is a corrupt file on the Mac.
Can you partially sync? Start by syncing no music, then add playlists until you get the error. Once you find which playlist it is in, watch the sync to see if a particular file causes it or once again shorten the playlist into segments until you can find the track that is causing it.
If you get the error with no music, then it could be a photo, or a document. Repeat the steps to reduce the sync, increase to find the file.
If you get the error with no files coming from the Mac, then it could be the backup is corrupt. Try unencrypted backup to see if it goes away. If it does, turn on encryption again, and that will replace the old backup with a new one.
If you still get it with the unencrypted backup, try not backing up. If you still get it, it could be a log on your HD that is corrupt.
You could wipe the logs, but we'll look at those steps if you get here.