well it is documented in the GUI:
Disable webGUI anti-lockout rule
By default, access to the webGUI on the LAN interface is always permitted, regardless of the user-defined filter rule set. Enable this feature to control webGUI access (make sure to have a filter rule in place that allows you in, or you will lock yourself out!).
Hint: the "set LAN IP address" option in the console menu resets this setting as well.