Unable to Restore iPhone 4 and ERROR -50
This error (-50) usually happens when the location iTunes is trying to use for the backup is taking a long time to respond and times out (network share, iSCSI, USB/external storage...). If this is the case, the solution is to temporarily move iTunes backup/download location to your harddrive.

Here's also a KB from Apple:


  Error 13 and 14: These errors are typically resolved by performing one
  or more of the steps listed below:
  
  
  Perform USB isolation troubleshooting, including trying a different USB port directly on the computer. See the advanced steps
  below for USB troubleshooting.
  Put a USB 2.0 hub between the device and the computer.
  Try a different USB 30-pin dock-connector cable.
  Eliminate third-party security software conflicts.
  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.
  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.
  Try to restore from another known-good computer and network.
  


[...]


  Error 50: This error (and error -50) are timeout errors. Follow the steps for errors 13 or 14 above.


Regarding recovering data from backups, I used: http://code.google.com/p/iphonebackupbrowser/ to look through the app's data.