iPhone backup or sync issue "unknown error occurred (-50)"
I got this error just now. My Mac mini was connected to my LAN via WiFi and also via Ethernet. That was working ok, but I was trying to get the phone to sync via Wifi (still can't get that working). 

Anyway, I pulled out the Ethernet cable. Plugged the phone in via USB and when I clicked Sync I got the (-50) error you describe.

My solution was to exit iTunes, and then launch it again. This was enough to get the phone syncing with no error.