I sometimes use light screen manipulating apps on my galaxy s4(such as twilight app,...) and when the apps are enable,in my vpn apps,ok button in vpn allow connction dialog does not work(cannot be tapped) but cancel button works az usual!
I want to know what is the behind?i think this is not a bug, it is something like a rule.
Your security is the reason
This is to prevent Tapjacking . See bmdixon's answer here Screen overlay detected, what is the problem? for details
So, VPN is not permitted but if you don't have a Cancel option to go back ( and disable the screen app, normally option for this is available in app settings) - how do you go back ? You can't, therefore, Cancel option is NOT greyed out
Q & A