I am getting the same "Unknown error (-50)" when trying to sync photos from my Aperture 3 library now.
I did not upgrade to Lion (waiting for a 3rd party driver I need to be updated first for it), but I did update iTunes to 10.4 (80) recently and that is when I noticed the issue.
I haven't had this issue before. It syncs everything else but when iTunes goes from "Analyzing Photos" to "Optimizing Photos" in the sync process it reports that it is unable to sync because of an "unknown error (-50)"
To resolve the problem, I tried the following:
- I repaired my Aperture library database (using Option+Command when launching Aperture 3).
- Deleted the "iPod Photo Cache" folder inside my Aperture library. It had to resync all my photos again, this time it worked, but the next sync it had the same error again.
- Updated my iOS to the latest version.
- Did a Repair Disk Permissions on my Mac.
- I changed USB cables and ports (using only ports directly on my Mac Pro).
- Tried syncing with only iTunes running.
Doing all the above made no difference, I am still getting the error message and it won't sync new photos back to my iPhone 4 now.