IWP Instant Web Publishing starting but won't change port

[jn08]

I'm using Filemaker Pro 11 and trying to switch on IWP. I'm using Mavericks and Server on a mac mini in our office. IWP switches on after a long time with a spinning beach ball, but even though it will allow me to change the port number to something else when I re-open the preference pane for IWP the port has changed back to the default (80).

 

When I try to access the server from inside or outside my network I get a "No data received" error. Anyone have any clues to fix this? I'm sure I had IWP running when I first set up this database, but I have no idea why it's suddenly not working.