The cause of this issue is that packets are being dropped. The fix is to create a new DWORD key in the Registry of the VM that is failing to backup. The Registry path is \HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Tcpip\Parameters, create a new DWORD with the value of SynAttackProtect and set the value to 0.
The following link outlines the function of the SynAttackProtect http://technet.microsoft.com/en-us/library/cc938202.aspx registry value. It is a protection mechanism against TCP SYN's (handshaking) floods, setting the value to 0 disables this features.