Panther server FTP "command not understood"

I have set up FTP on my Panther server (10.3.2) and all seems to work fine ... except: The first command after login always gets the error: 502 'EPSV': command not understood. The command still runs but it takes a long time (minutes) before the output is displayed. Has anyone else encountered this problem and/or know what is happening? TIA.