I've created a new FTP account for a website and I can't login to it. I have other FTP accounts that are working just fine. And I've looked in "/var/log/auth.log" and it says "user unknown".
Are you sure that you connected with FTP or FTPS and not SFTP? SFTP is not FTP, SFTP is SSH (the SSH File Transfer Protocol) and SSH requires a shell user while FTPS is secure FTP (FTP over TLS) and uses an FTP user.
I discovered the problem and fixed it. I used phpmyadmin to look in the "ftp_user" database and, somehow, there was the same FTP user - the one I wanted to login as - in there twice. Pure-ftpd was seeing the first stale user and rejecting the login. Basically, the FTP user list had become out-of-sync. Though don't worry, as I think I'm responsible for that, not ISPConfig. Because I did reorder my cluster - make the nodes all mirror the storage server, rather than the first web server node - as part of my fix for the earlier FTP problems (which essentially was having the FTP by-pass all my load-balancing and be port forwarded directly to the FTP server - I will eventually have to revisit that problem if we expand further and need more than one FTP server, but this fix works for now and FTP is a low traffic service, so it's not so important to load balance that yet).
