Sometimes it looks like IPERF Server doesn't start or starts but produces no output when Client is transmitting.
Windows Task Manager shows incoming traffic.

When clicking Start / Stop / Terminate / Exit too often, process may hang and socket gets stuck.

Check open sockets, for UDP:

netstat -anp udp

or for TCP:

netstat -anp tcp

You will see that the socket which Server wants to open, is already opened by previus iperf instance. Its window is already closed, but socket is still occupied. You can even run client traffic to that hidden server, but it won't show up in the last launched server window, because it can not open the server socket.

Use -o and -b netstat options to find out pid and executable name for that port number. Then Tcpview.exe or Pskill.exe from SysInternals to terminate that process. IPERF Server starts working on that port number..

The latest IPERF version auto-detects listening socket on a port number, which is going to be used, and brings up the warning message.

Be patient with sockets, they open and close slowly and don't like fast-clickers.