Hello
Although this is a year later it may be useful to someone.
I have just had a similar problem and it was caused by the IP address returned in the set passive mode response being invalid:
[=>FTP] “227 Entering Passive Mode (x,x,x,x,198,245).” where x.x.x.x is a address unreachable by the modem.
If the FTP server replies with an internal network IP to a unit that on the internet then it is unlikely it will be able to connect to the opened port.
Chur