After exposure, ERR20 – shooting is not possible message. This means some mechanical failure.
Official Canon advice:
Description: A malfunction with the mechanical mechanism has been detected.
Resolution: Turn the power off, then remove and re-install the battery. Then turn the power on again.”
Or say a prayer, whatever.
Reading out with SPT software:
This means, the mirror position sensor is not working. I disassembled the camera, and on the flex cable which comes from the sensor, I have found a tiny oxide dot (or what).
I washed it away with normal water, then I treated it with some methanol, to take away the water.
And now it works, and no error code!