Plumbing – New Rheem RTG-84-XLN-1 Error 12

I just installed a new Rheem RTG-84-XNL-1 tankless water heater. The unit has been throwing error code 12 off and on since it was first run. The manual and their tech support only entertain the idea that there is insufficient gas supply. Here are the numbers:

• 10 wc from the meter
• 10 wc static at appliance
• 1 to 2 inch wc drop when unit fires
• electrical is well grounded and unit powered via a disconnect fed by a home run
• gas and water bonded
• burner is running and I can see string blue flames when the error gets thrown
• burner tubes look good. Wiring is tight and rods aren't cracked
• unit specs require 4 wc min and 10.5 wc max (as per the machine panel)

Two things that may be worth noting:

• there is water accumulating in the bottom of the chassis. At first I thought this was a leaky internal component but after further inspection it appears to be condensation
• it seems like it tends to error out when particular fixtures are being used. For instance, every time I have turned on the master bath shower it has worked fine – another shower (also upstairs) always seems to anger it.
• the house is two stories with a split level basement with the unit installed at ground level attached to the exterior of the basement at a location where the basement is level with grade. All that to say that the hot water runs are pretty long.

The Rheem tech did say that the unit requires < 1inc wc drop, which contradicts both code and their installation manual, which states:

The gas system is designed to operate at a certain
maximum pressure drop. A pressure drop greater
than what is permissible can cause operational
issues with the gas appliances. The National Fuel
Gas Code (NFPA 54, ANSI Z223.1 2012) allows
for three pressure drop levels, a 0.3 inch W.C.,
(see table 2); a 0.5 inch W.C., (see table 3) and
a 3.0 inch W.C., (see table 4) pressure drop for
natural gas

My inclination is to demand they send me a new unit. Is there anything else I should be looking at? Is this condensation (or what seems to be) normal? Could the water plumbing somehow cause this?