Some users have discovered that the keyboard does not open as expected when trying to enter their PIN or password to unlock the phone.
Many assume that this could be due to the problem of the dead zone of the screen, a small area that seems not to respond. The solution is actually much simpler and basically involves enabling a configuration that should have been activated by default.
Go to Settings> Applications and open the Advanced Settings menu by touching the three vertical points in the upper right corner. We touch on Show system settings and scroll down to Samsung Keyboard. We go to Advanced Settings and grant permission for "Applications that may appear at the top". This should solve the problem. The permission must be allowed by default, but it may not be allowed in this case.
This permission is required even if you install a third-party keyboard.