“Screen overlay detected” dialog? Turned off everything, doesn't work
There are three places to check. Mine was caused by the Forest app under Acessibilty.

Non-system Apps

Settings -> Application Manager -> Apps with Overlay Permissions -> Apps with Permissions

System Apps

Same as above, but with Show System Apps turned on

Accessibility Apps

Settings -> Acessibility -> Scroll to the bottom. It lists app names. Just turn them all off for now.