The server is unable to listen on a socket. The backlog refers to the maximum number of queued connection requests that may be pending at any given time. Check the following: 1) The machine is still connected to the network, and the network is responsive. 2) There are no firewall or other restrictions preventing a socket listener from running on the current machine. 3) The backlog setting is within the limit, if any, on the machine. 4) There are enough system resources available. Free up system resources if they are running low.