A high pressure alarm can sometimes occur due to solar gain which is where a portion of the waveguide is exposed to direct sunlight during the heat of the day. The air inside the waveguide expands as it gets hotter causing a rise in pressure.

You can confirm this by turning off the dehydrator, bleeding off the pressure and restarting the dehydrator. If the alarm is due to solar gain then the system will pressurize to the correct level and stay there for some time. If the alarm is due to an issue with the dehydrator then it will over pressure right away.