Judge server blocked by firewall on OSX 10.8.4

I'm unable to get remote network access working while I'm running as a judge. This is under OSX 10.8.4 on a Mac Pro with d20Pro v 3.3.4:

1) I've verified that the port forwarding on my router is working by having a different app listen on 10101 - worked
2) I've verified that the d20pro server is running by telnet to 10101 at localhost - connected to something, characters to terminal then a disconnect on hitting enter
3) I've verified it as an OSX firewall-related network problem by turning the OSX firewall off - it then works
4) I see an Allow/Deny dialog box from the firewall pop up briefly as the main window opens, but then it vanishes without being clicked. It isn't behind the main window or hiding on another screen.
5) The app has been added to the list of allowed apps in the firewall pane of the system preferences
6) I've also manually added the app to the firewall in the terminal:
silver: djb$ sudo ipfw list
Password:
07000 allow tcp from any to any dst-port 10101
07001 allow udp from any to any dst-port 10101
65535 allow ip from any to any

So something during the app startup causes the main window to appear over the firewall dialog and either permanently hides it or dismisses it with a 'deny' response before I can do anything about it.

I have a screen capture movie of it happening if that helps, see this movie

Any hints or tips greatly appreciated. Turning the firewall off entirely is a workaround, but not one that I want to use routinely.

 

For the hell of it, updated to Java 7. No change in behaviour.