Hi, I'm getting exactly the same issue, started this morning. Opening the phone in Safe Mode makes no difference. If WIFI and data are disactivated and the phone is then switched on the white rectangle is no longer visible and the screen is accessible. When the rectangle is visible then I am unable to access any of the apps currently running. However, I can still drag the status bar down from the top of the screen and read the notifications. If I click on a notification it will open the associated app, but will display the rectangle on top and not allow me access to the app.
I am using a Samsung Galaxy S6 with Android version 7.0.