Symptom: This FTP server has been running great for a while. All of a sudden it stopped. Now, when trying to start, you receive the message "Error starting the server. Make sure that there are no port conflicts." Checking the log, you see "Unable to start socket server."
Cause: Microsoft Update turned on FTP Publishing or IIS services on your computer; or after running a Microsoft update a new service becomes active, cliconfig32.exe, which "Provides TCP services and allows Microsoft Support to Help You on this computer."
Resolution: Run the netstat -a -n -p tcp -b at a command prompt to see which service is taking over port 21. Turn off the service that is activating port 21 on your server. Please make sure that there are no FTP services running under IIS and that the FTP publishing service is not turned on via your services.msc console.
For more information on checking ports, see our article: http://support.southrivertech.com/solution/articles/9000023975-how-to-determine-which-program-is-using-a-given-port