Tried several times on an IPV6 only server and a dual stack server, found it might be IPV6's problem, which showed connection refused by server. Server uses pureftpd 1.0.49, and client tried Filezilla, FlashFXP and others.
I'v tried to set ForcePassiveIP in the config, and open the ports in the ufw, and use plain text password in Filezilla, none of these worked. Any solutions?