Can’t edit firewall rules in Server Admin

10.3.9 Server

When I set the machine up I managed to add some “advanced” server rules, no problem. I last changed these a few months ago and it has been happily sat there since then, working away with no problems. Now, if I try and add or change an “advanced” rule I get a spinning cog wheel for ages, then it relaods the previous state (ie ignores my changes) and turns off the firewall.
Things I have tried which make no difference:
– authenticating as different admin users
– disconnecting the server from the network and turning off the forewall service before making any changes

I’m out of ideas, but I need to be able to edit the firwall setting.
Any help much appreciated!