There was always a x476dw false ADF paper jam error. - Clean ADF with air did not help. There was definitely nothing. - Remove ADF unit showed the same error also with disconnected ADF. - Also a complete factory reset did not help.
Solution: You can see a main board if you open the back cover . The main board contains a CR2032 battery. I replaced that battery with a new one. And the problem disappeared.
Not nice: I assume that the the printer lost something which was not cleared by a factory reset. The developers should work on such problems.
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