iPhone 5 cannot be synced. Unknown error occurred (-50)

You should have an issue with securtity software.

Try this: http://support.apple.com/kb/TS1583

Do you have any of the 4 types of software on your computer?

It dos not matter if it is enabled. Having installed it is sufficient.

Try switching of your OS internal firewall, temporarily uninstalling security software ?

Ensure that no software interrupts the contact to the named servers.

There is another Apple support doc for this error.

Error 13, 14, 35 and 50 (or -50)

These errors are typically resolved by performing one or more of the steps listed below:

1. Perform USB isolation troubleshooting, including trying a different USB port directly on the computer. See the advanced steps below for USB troubleshooting.
2. Put a USB 2.0 hub between the device and the computer.
3. Try a different USB 30-pin dock-connector cable.
4. Eliminate third-party security software conflicts.
5. There may be third-party software installed that modifies your default packet size in Windows by inserting one or more TcpWindowSize entries into your registry. Your default packet size being set incorrectly can cause this error. Contact the manufacturer of the software that installed the packet-size modification for assistance. Or, follow this article by Microsoft: How to reset Internet Protocol (TCP/IP) to reset the packet size back to the default for Windows.
6. Connect your computer directly to your Internet source, bypassing any routers, hubs, or switches. You may need to restart your computer and modem to get online.
7. Try to restore from another known-good computer and network.

You might have recived your computer with security software preinstalled (McAffee is common) in any case your operating system has a firewall installed, that can be disabled.

this solved my problem too.....many thanx.