Same problem

First make sure your your installation phasing is correct. You can easily do this by first checking the power at the micro wave socket as explained below with the gen. or land power and then see if you get the same reading to ground connection to connection with the inverter on if it is different swap your input AC from your inverter to your power grid

If that is OK

Check to make sure your microwave socket and or/ the device plug are phased properly to ground. The white wire should have 0 ohm continuity to the device ground. The wall socket should have 110vac between ground and the black(small hot side) wire and 0 vac to the common (big white) side.
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