In the USA electrical system, the Neutral wire is referenced and tied back to the Ground wire in a residential Mains circuit panel for the wiring in your house. The 120vac wall outlet has a wide slot, or sideways-T slot, on the left for Neutral (white wire), a narrow slot on the right for Line or Hot (black wire), and a roundish hole at the bottom for Ground (green wire).
The charging EVSE unit and/or the on-board charger of the car are looking at the input lines to determine that this is true for the connection.
The generator is making an AC voltage that is floating with respect to Ground--the Neutral is not connected to the Ground. Checked the generator manual on page 73 in the technical information section and the diagram shows no connection.
So a male plug can be used to make a jumper to insert into one outlet such that the charger will be happy on the other outlet. This requires a trip to home depot.
In the backshell terminals of a male connector plug put a 12" piece of copper wire (green if you have it, any size guage 14 to 18 AWG) connecting the screw terminal for the Wide blade to the screw terminal for the Round pin. Let the loop of wire extend out the backshell to serve as an indicator flag, and to hang on the wall when not using it.
On the Mains in your house the terminal labelled Ground is actually tied to a wire that sticks into the dirt of the Earth. The dirt or ground of the Earth can conduct electricity and acts as a free wire for the utility company--so the dirt is used as a safety wire path back from your house to the center tap of the transformer on the pole, or pad if you have underground utilities.
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