Failure in login doesn’t have to do with the firewall. If you can see the login screen, then the firewall is allowing the traffic. However what may happen is to see the login screen cached, in which case you cannot understand that it is blocked by the firewall. Best solution is to watch the counters of the iptables with the command
iptables -L -v
to see if the blocked packets or the accept packets raise when you try to connect.