To see what is or is not listening on port 5000, use this command at a Terminal window near you. lsof -Pn | grep LISTEN On the right you'll see TCP *:5000 (LISTEN) usually listed once for each IP version, and on the left is the name of the program that's sucking on the port, which is ControlCe if its AirPlay. If nothing is using that port, then it won't be listed.

POSSIBLE WORKAROUND is to remove the power cord to the monitor and plug it in again, whilst still connected to the Mac. Standby on/off cycling doesn't work. I'm new to Mac, having used Windows since 1989! Funny as the only problem I've ever noticed in forums on Mac is how they have problems detecting external displays, so I guess I'm not surprised it's happened.