When I meter gnd against 5v on the adapter, I get 5.15v. So both the adapter and the DMM are fine.

I heard that connecting two DC supplies together would have no problem if they both output exactly the same voltage. Of course we would never have an exact match, but the only strain on either side of the connection would be from the difference between the 5v from the adapter and 5v from the washing machine. So I’m tempted to conclude the serial port was sabotaged at the factory.