Cannot open '/dev/video0': 11, Try again #23

I get

NativeWebcamJNI: Cannot open '/dev/video0': 11, Try again
and then
NativeWebcamJNI: Unable to load frame, buffers not initialized

what is 11? why try again? I'v tried...

p.s.
Android 4.3, rooted
permissions are fine:
crw-rw---- media camera 81, 0 2017-02-19 04:11 video0

Anybody help