Hi konung,
Try this: activate the helper on both WAN and LAN interface. Add a rule on the LAN interface allowing all traffic, all ports to 172.0.0.1, make sure this entry is the 1st in the list.
It solved my problem with ftp connections (multiple WAN). Connections setup by Inet browser,ftp client and cli are all working now.
If you need incoming sessions as wel you'll need to add NAT rules to allow it.
regards