Cannot connect the camera!

Hello! I'm fresh in here, welcome everyone ;)
I've a weird problem with my Galaxy S4. :/ The thing is that the camera doesn't work. What is strange about the issue is that when I turn my mobile off and on and turn on camera or snapchat ... the camera works! I can take a pic or record or whatever. Nevertheless when I attempt to switch the camera from the rear to the front, the screen turns blank and after a few seconds what I can read is 'Warning! Cannot connect the camera!' and it turns off. The same when I turn the app off and on again. It works only when launched the first time after turning on the phone. This facr indicates in my point of view that the problem is software - not hardware!

I bought my Galaxy S4 in the UK, it was from Three (H3G). I am from Poland anyway, if it makes any difference :D

When I dial *#34971539# I can see this info:
Phone/CAM FW Ver Chceck:
Rear Camera
Cam FW Ver: 013F0LAGI02
Isp FW Ver: 013F0LAHA01
Phone FW Ver: 013F0LAHA01
Front Camera
Cam FW Ver: S5K6B2YX
Phone FW Ver: S5K6B2YX

ISP Ver Check:
Rear Camera
ISP Ver: M10M0
Front Camera
ISP Ver: S5K6B2YX

FW Write Count:
Rear
(0:2)
Front
(0:0)

CAM FW Cal Check:
Rear Camera
null
null
Front Camera
null
null

ISP Core Voltage Check:
Rear Camera
ISP Core Voltage: null