Xperia Z5 :: Can't Accept Calls On Phone

While using the phone (don't matter if in a 3rd party app or in native or standby home screen), and somebody calls me a little pop up with the incoming call appears om the top of the screen (DENY/ACCEPT). 

Now it comes: it doesn't matter if i tap on acept or deny call, the phone doesnt react! It continues to ring, and sometimes after tapping several times it does what it should do. 

The problem is not the display, as it works fine everywhere else, and the phone app itself should be ok as i can receive/deny calls when it appears in the full screen while in idle mode. I think the problem is that little pop-up GUI. Tried to reset the phone to factory reset, but it does the same also if the phone was unsued before!