Even if the checkerboard is detected, it needs to fill the majority of the non-shaded region. In order to progress to the next step, you need to bring the camera / pattern closer together and then the procedure will accept it. I know this is not obvious when you do it for the first time.
Regarding voxl-camera-server freeze, what if you run voxl-inspect-cam <camera_stream> on voxl2, does it still show images coming in?
Try setting your voxl into perf mode (voxl-set-cpu-mode perf) which will speed up the calibration pattern detection. I see that the calibration is only running at 2.8FPS at 1280x800 resolution.
During calibraiton, you may also want to stop other services that might be doing image processing, so that VOLX2 has more cpu cycles for the calibration app.
Alex