there is a bad implementation of the Proxy's parameter 'USER after logon' :
Filezilla tries to connect using :
USER user@server:port
and gets the server:port cannot be found...
That's because the USER syntax is USER user@server port (no ':', just a stupid space)
That's not a very important bug because it can be bypassed by telling the port is 21 and the server's address is "address port" ("192.168.1.1 2121" for example)
(and please forget my awfull english
)
)