I´m having the exact same problem as Breezer here.
I have created an ftp user under a client and that user is marked as active. When I try to connect via filezilla, I get the same error messages as posted earlier in this thread. I can however connect as root, even though root is present in ftpusers under /etc
Here is what pops up in the auth.log:
sshd: Invalid user **** from 192.168.255.3
sshd: Failed none for invalid user **** from 192.168.255.3 port 53982 ssh2
sshd: pam_unix(sshd:auth): check pass; user unknown
sshd: pam_unix(sshd:auth): authentication failure; logname= uid=0 euid=0 tty=ssh ruser= rhost=192.168.255.3
So obviously the ftp does not recognize the user I created in ISPConfig, so the question is why not?