Chessboard calibration did not work

I tried to do the chessboard calibration, but the system reported an error. Here is the screenshot of the app:

What can I do?

The 3 images are the control images that are shown after each chessboard calibration attempt. The left image is taken from the left camera (when standing behind RyngDyng and facing the target), the image in the middle is from the right camera and the right image is taken from the middle camera.

From the images we can see that the orientation of the cameras towards the center of the target is not optimal. The view of the cameras is too low and therefore the chessboards appear too high in the image.

If the chessboard calibration is successful, all three images will show the chessboards centered in the control image. However, this is the case only in the rightmost image, the other images show a failed calibration and the red frame indicates the location of the problem.

The right camera (=middle image) has got a view where the upper chessboards are no longer in view. Only the middle right camera (=right image) has got an optimal view on the chessboards and therefore the chessborad calibration worked for that camera.

The solution is to make the cameras steeper looking up. This can be achieved with the help of the adjustable stands of the RyngDyng case. Try to lift the front of the case and then take test images showing the camera center with a red hair cross. The following image shows a situation where the alignment of the center of the cameras with the center of the chessboard squares is very good: