The server uses ISPConfig 3 and the following Lines appear in the Syslog every 5 minutes:

Jul 22 15:15:01 my pure-ftpd: (?@127.0.0.1) [INFO] New connection from 127.0.0.1
Jul 22 15:15:01 my pure-ftpd: (?@127.0.0.1) [INFO] Logout.

These logins are from the ISPConfig 3 system monitor which checks pure-ftpd every 5 minutes. So these lines do not indicate a problem on a ISPConfig 3 server.