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.