One of the theories for why we're experiencing this problem is buildup of condensate in the flue that exhausts hot air from the Navien. The flue piping has a long level section that elbows up (see photo) and water could collect here in the winter, since outside air will be cold and make the pipe cold. If enough water is collecting there, it may be blocking exhaust enough so that the system senses an air pressure problem. We've been able to reboot (unplug and plug) the system and get it working again.
We hesitate to go in and realign the exhaust pipe because it is housed in a chase for cosmetic reasons (see photo below).
Matt disconnected the intake pipe on the Navien so that warm air is being drawn from inside the home (see photo below), rather than outside. We'll see if this fixes the problem when the weather turns cold. If not, we'll have to cut a hole in the chase where the vent pipe is located and attempt to incline the pipe and exhaust it directly under the deck instead of on the roof.