@videoguy - Now that the error domain has been reduced to the master valve, here is what I’d try.
Move the Master Valve wire to zone 11 and run zone 11 manually. If there is a wire fault reported on zone 11 (my bet) then there is something up with the wiring to the master valve.
If there isn’t a wiring fault reported above, then I’d take a zone wire from a zone that didn’t fault when the master valve was disabled and place it in the master valve terminal, with the master valve setting turned on in the application and perform a quick run on that zone (only one wire actually being energized as that terminal will be empty as its wire is in the master valve terminal. If a wire fault shows up in this configuration, then I’d call Rachio as there may be something up with the unit.
However, I think fire ants, squirrels or rats may have chewed on the wires going to the master valve causing a short. I have seen fire ants all over electrical connections before - something about it attracts them. It could also be the actual master valve solenoid that may have shorted out.
In the mean time you could leave the master valve manually open to run the system (without the master valve activated in the app) until you get it fixed.