Thanks. Forgot to mention that I did have a loaner MX with its own mobile connector while they were troubleshooting mine. Same issue with the loaner MX / mobile connector.
That's telling, I think. Combined with your car charging fine at the Service center, it clearly says there's something about your connection that Tesla's programming doesn't like - something that apparently changed a few months ago.
I don't have any bright ideas what that might be, unless it's line noise like folks have been speculating above. See if you can get a power conditioner for a decent price or as a loaner to try out, maybe?