I'm using v2.6.4 of WordPress Simple Firewall and trying to activate Admin Access Restriction. I check the Enforce Admin Access Restriction box, specify a plugin access key, and hit the Save All Settings button. The page reloads with a form inviting me to enter my Access Key. However if I select any of the Simple Firewall links in the left-hand admin menu in WordPress I bypass the Access Key login and the plugin page loads normally. Then if I visit the Simple Firewall dashboard I can see that Admin Access Protection is OFF.
I've repeated this several times including with a different web browser and I get the same result.