I have a similar problem. I everything works well with IWP from inside the local network (using 192.168.0.x addresses), but once I try to access the server from outside the network, I can see the database, and get to the login screen, but it hangs at login.
I've tried logging in with several accounts and nothing gets past the login screen when using the outside IP adress.
Obvisouly, I'm getting TO the file on the server since I can see it and get to the login screen, so it doesn;t seem to be a port forwarding issue... filemaker just won;t let me login to the datbase when accessed from outside the local network.
Is there some kind of advanced authentication that's failing? I'm using FM 8.5Pro