FTP Stopped Working

I'm trying to figure out why my DOpus FTP functionality has stopped working properly. From looking at the log it seems the login step is ok but then getting a directory listing results in:

150 Opening BINARY mode data connection for /bin/ls.
425 Can't open data connection.

This happens on both Windows and Unix boxes. I've turned off my software firewall and tested another FTP client which worked fine. Also SSH FTP works fine. I'm using

Try turning on PASV (passive) mode in the FTP settings.

That did it. Thanks.