I had the same situation, I mean when I tried to install any apps with unknown source I had active cancel button and inactive install button.
After wipe and install clean rom everything was OK so I was sure that issue was by my installed apps. In this case: Lux Auto Brithness. After freezing this apps - install button is active.
Screen filtering apps causes this issue.
I believe they add layer and android disables INSTALL considering security..
If clearing data for media storage seems like temporary fix, then no permanent fix(?) is there instead removing the app thats causing the screen overlay (Unless devs find one and they will)
In my case it was EDGE MASK app's rounded corners effect that was causing issue.. Disabled it and worked..
Google for screen filtering apps and find your's..
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