The shibd daemon generally uses a socket to communicate with web server processes. A "socket in use" error will prevent the daemon from starting. Usually it happens on Unix/Linux when a Unix domain socket is being used. The domain socket is a special file and if shibd shuts down uncleanly the file may be left around.
To fix it, you can remove the socket file (see ShibbolethXml in the <Listener> element for the file path), but you can also start shibd with the -f option to forcibly remove the socket.