[update 21.03.17]
I started to get black viewfinder on my jolla C...
I don't really know when it started, now I'm on snowboard vacation and when wanted to take some pictures, I noticed about black viewfinder on main camera. On front camera it sometimes work and sometimes also black.
I didn't install any new things, but I did 2 new things:
1. I changed battery, that work perfect.
2. I now in French alpes with with roaming from my Israeli sim card. The connection dropped many times 2-3-4G, sometimes lost connection and reconnect again. Sometimes I feel a hot spot around the camera lens.
That all..
Already tried:
1. killall -KILL mm-qcamera-daemon = not help
2. Reinstalling camera app = not help
3. Removing camera app + reboot + reinstalling camera app = not help
4. Disabling all patches = not help
5. Lipstick restart = not help
6. Reboot/shutdown - sometimes it started to work, but after some minutes I get the black viewfinder again. Today even reboot/shutdown get the same result (black viewfinder) = not help
7. Tried to play with dconf commands related to jolla-camera, but didn't find something that I can change for fixing this problem. Also reset not help..
8. CSD tools camera tests - sometimes pass for both cameras, sometimes only for front camera..
9. Tried to install Camera Plus from openrepos, but for some reasons it not starts at all.
10. I also tried some different android cameras and all of them identified only front camera.

Any other ideas for fix ?
Thanks.