My 12 Volt battery died at -40C. Had battery tested, it was dead. Replaced battery and now have error message "12 Volt Battery charging system problem". Car will run for a few meters and then error appears as the 12 V battery voltage drops as presumably it is not being charged. I recharge the battery with a battery charger and the cycle reappears.
Have you measured the voltage with the car in Ready mode? It should be around 14V. If not, there could be an issue with the DC/DC converter (fuse, relay, wiring) which charges the 12V battery from the HV battery.