You probably know about the Setup–>https tab–>Allow access only from (IP or subnet) settings. Of course, if these get set wrong, you can’t get in to fix it.
Have you tried this: (from the FAQ)
Having incorrectly configured the firewall or the network in general I can no longer connect via the web interface. Even rebooting the system doesn’t work. How can I resolve the problem?
From the ZeroShell console press the Z key. This enables the system to start the Fail-Safe procedure: you will be requested to select an Ethernet interface which can be attributed an IP address of your choice; any obstacles, such as firewall rules, belonging to a bridge, static routes or other will be removed. Once complete, you can access the interface via the IP address you attributed. To exit the Fail-Safe mode you must reboot, having removed the obstacle that blocked access.