Try turning off the firewall or other security apps. Also make sure make "DHCP Client", "RPC Endpoint Mapper" "Remote Procedure Call (RPC) Locator" and "Remote Procedure Call (RPC)" services are all running and set to automatic.
Just to eliminate, reboot the computer once, then try backing up.
Also, you should have these registry entries:
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\services\RpcSs
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\services\RpcLocator