It's amazing how many different errors this -50 seems to represent. I was having the same problem as you. I followed all the Apple suggestions in this community and nothing seemed to work. The last thing I tried was to simply create a new library and reimport my music directory into iTunes and everything works fine. I can now copy mp3's from amazon, itunes, or CDs onto my iPod. Just be careful about the directories your music is in. I had Amazon put my mp3s in My Music folder. iTunes puts them in a iTunes\iTunes Media\Music. I think the new version of iTunes gets confused between icloud and the other directories.
Hope this helps others with this problem.
Thanks for the information. I actually fixed my problem after many hours of experimentation in two different ways.
I found that when I filled in more information in the catalog about songs on mixture albums, I had to always check the COMPILATION box to have those songs sync correctly. Apple didn't say anything about that. Without the checkmark on each song in the compilation album, the song would cause a 50 error.
I also found that if I moved the problem songs (that caused the error 50 message) out of my Itunes Library and reimported them, that stopped them from causing the error 50. So I deleted them and reimported them into the library. I found that some of the songs in the Itunes catalog had showed weird locations with addresses like C:\\\documents and settings...instead of the real pathway that might have been C:\documents and settings.. Some pathway addresses had become corrupted for some reason in the information that my Itunes library stored. So when I reimported them into the ITunes library from another location, their pathways were fixed and they no longer caused the 50 error while syncing. I painstakingly identified each song that was failing and deleted each specific song to clear up the problems.
That said, I do appreciate this board and the fact that others are willing to take time to help other users. Much obliged.