|I found out that the sensor at paper loading tray (MP460) is much too strong than it should normally be.|
If you leave the printer without paper for a long time (even a few hours) it (sensor spring)can push paper back and give 5100 error.
I filled the tray with as much paper as possible and in fact pushed it manually with my hand backward, so that paper rests firmly in the tray. Then powered up and it worked.
It works fine all until you leave it empty next time.