Or could this be due to a wiring problem inside the house that I should
check first? Or perhaps a failed circuit breaker in this subpanel?

My neighbor had that problem in his 20 YO house.. One or more neutrals in
his Service box were loose. He "lost" half of the appliances in his home:
stove fridge and micro got "cooked". The "fix" was to tighten all the
neutrals with a screwdriver