What did you find out?
I have the same problem, starting 6 months after the 14-50 was installed and started using it. I've got #4 AL wire to the outlet in my garage; when it first started happening, around June, I managed to get it to function by dialing down to 20 amps. Now even that causes the breaker to trip. The outlet and UMC are cool to the touch, but the breaker is very warm. During warm weather it's bearable - I can charge at work or a nearby SC, but I like being able to prewarm the car in the winter. Plus, not knowing why it's happening feels hazardous.