I installed and configured proFTPd under Debian 9.3 as described under https://www.thomas-krenn.com/de/wiki/FTP-Server_unter_Debian_einrichten
however even after iptables -F I can connect with telnet, but not with FileZilla:
:~$ telnet 21 Trying ... Connected to . Escape character is '^]'. 220 ProFTPD 1.3.5 Server (Debian) 
Any hints on debugging FileZilla that go beyond the mentioned website?