Thanks for all the tips.
The camera have a 3.5mm TRS (stereo) “Aux in” jack. I have tested to swap the plugs from A->B and B->A in the board. Then the R signal gets stronger and the L is weaker (in the camera recording). So the problem is apparently something in the board, and not cables or camera.
We use the MainLR mix for this as we do not have any audience for the moment… because of Corona restrictions… and we do not have the main speakers turned on during recording.
I’ll try the test tone on the output jacks, to see if the signal strength differs on the jacks. Where do I find that function in the system?