Vsftpd, 503 Login incorrect, and so on?
So, you have vsftpd with mysterious errors, and you've checked your config 20 times, violated your PAM and passwd, yes?
And still no clue, yes?
And your user has some non-standard shell like /usr/bin/passwd, or /bin/false, yes?
Just add this shell to /etc/shells. Have a nice day.