can't transfer file between 2 FTP Servers

[MavFXP]

I'm trying to FXP between 2 computers, one running WS_FTP Server v5 and the other running Serv-U FTP Server 6.1.0.4.

I can successfully connect to both in FlashFXP but here is the error I get when I try and transfer a file from one to the other:

[R] Transfer Failed!
[L] CWD /users/Group
[L] 425 Can't open data connection.


Is there any setting that requires modification to transfer between 2 FTP Servers?

Thanks