Proftpd CentOS user can only login with a /bin/bash shell
Solution 1:
Any shell you want to use with proftpd should be listed in the /etc/shells file (http://www.proftpd.org/docs/directives/linked/config_ref_RequireValidShell.html)
You should add /bin/false to /etc/shells or set RequireValidShell off in the proftpd config
good luck!