Is that new furnace you installed shutting down on reversed polarity? Did you check, double check, and recheck the ‘hot’ and ‘neutral’ lines but still can’t solve the mystery?
Here’s a tip: Take a look at what other appliances are sharing that power circuit.
Sometimes loads, like appliances, can back feed power to the homes neutral.
The back feed voltage causes the furnace control board to ‘see’ power on the neutral, giving a reversed polarity fault code.
True story; I once ran into this issue and the offending appliance was an old 1940’s vintage desk fan. We unplugged the fan, and the problem went away.