Xperia Z3 :: Error Camera Not Available When Opening Any Application Which Uses Camera

My phone frequently returns a message "Error! Camera not Available!" when opening any application which uses a camera. This happens specially when the applicatons that scan barcodes are used ("AT&T Code Scanner", "OK Scan"), but not exclusively. Among the apps bringing the camera to a crash there are some of good reputation, like for example "Perfectly Clear". In the emergency mode the camera works OK.
 
The issue is the same as in this thread for Z1, however it disappoints me that the same happens in Z3 which is so much newer.
 
The support advised me to clear the data for the camera (Applications menu) and restart the phone. No dice, so they advised to reset the phone to factory defaults. I did it twice, due to after the first time I found exactly which apps caused the problem. It took me a day to restore everything to the original state .
 
Afterwards, the camera was stuck by "Perfectly Clear", a decent app written by a "Best programmer". I was depressed.
 
However, there is a solution: simply clearing all the apps running in the background (["three permanent keys in the bottom" | "two rectangles" | clear all], or [an icon with a green brush]). This works without a restart.