You were probably using a TT-30 to 14-50 adapter for RVs. These are wired differently in which it puts the hot wire onto both hot pins of the 14-50 and the neutral to neutral, so the UMC sees 0V across the hot pins and throws an error. This works fine for RV's because they use the two hot wires...
I will be visiting a campground which I may have access to two TT-30 outlets (on separate non-GFI breakers). Has anyone used or know if a basic Y splitter 2xTT-30 to 14-50 adapter would be supported by the Rivian mobile charger if I were to ensure the two legs are opposite phase?
Obviously I...