Only one usage of each socket address (protocol/network address/port) is normally permitted (76577)

Backup fails with the following message:
Only one usage of each socket address (protocol/network address/port) is normally permitted.

When a client initiates a TCP/IP socket connection to a server, the client typically connects to a specific port on the server and requests that the server respond to the client over an ephemeral, or short lived, TCP or UDP port. On Windows Server 2003 and Windows XP the default range of ephemeral ports used by client applications is from 1025 through 5000. Under certain conditions it is possible that the available ports in the default range will be exhausted.

Check the MaxUserPort key in the registry.
If this key does not exist create it like this:

• HKLM\SYSTEM\CurrentControlSet\Services\Tcpip\Parameters\MaxUserPort
• REG_DWORDMaxUserPort
• Make this a Decimal value of65534

http://msdn2.microsoft.com/en-us/library/aa560610.aspx