I did a fresh install of ISPConfig on Fedora 8, following the Howtoforge 'Fedora 8 Server Setup: LAMP, Email, DNS, FTP, ISPConfig'. After creating a client and site, I was not then able to FTP to the site, and kept getting login error 503. I had previously had it working OK on another box, so didn't know what the problem was. I searched forum after forum, and none of them solved it. I finally solved it after simply uninstalling and re-installing the proftpd package using the following steps: yum remove proftpd yum install proftpd chkconfig --levels 235 proftpd on /etc/init.d/proftpd start The last 2 lines ensure that proftpd starts automatically when the server is rebooted. This fix may not work for everyone, but for those with the same problem as me, it's a life-saver. I think ISPConfig adds some dodgy lines to the proftpd.conf file, and a re-install does the trick. Tuyre.
This same problem pagued me for two days, and I searched many forums and read many replies to the problem, but this is the only one that actually said what to do, and best of all, it worked!