It turned out that Scanner cover was indeed "open". Unfortunately, the error message was not too specific: the guilty party was one of two hinges at the back of the cover (to be precise, a lower side of the hinge that is supposed to plunge into the body of the scanner) -- the one which was near the **_left side_** of the scanner. That part was not fully inserted -- just slightly off... Once I applied some force to it, it went down to a complete stop, and the status light changed from red to green. To bring the scanner to its normal operation mode, I had to turn it off and then turn it back on.