I don't know if this is causing problems, but you have
in your /etc/proftpd.conf. The first one is wrong, so I'd remove it and restart Proftpd.
Also there are many entries where a session opens and closes (about every 30 minutes). Is this correct?
This must be ISPConfig's monitoring script which tries to connect to your FTP server to see if it's still running. This is absolutely ok.