I had this issue in KitKat before (can't tap "Grant" in SuperSU popup when Root access is requested) and if memory serves, it was due to one or more apps creating a "screen overlay". I know it's caused by an app, but I can't figure out which one. Besides this, fixing it through changing an app won't stop it from happening again and again. It could even be caused by more than one app now.
I fixed it once a long time ago, but can't remember how. I thought it was by using an Xposed Mod, but neither of the ones that talk about "Overlay" fix it.
Does anyone remember how two fix it? Two days of searching XDA and Google have not produced the desired result.