Same year and model with the exact same issue. It started five days ago after arriving home from work. Did the two-button reset but the issue resurfaced.I got the same message on the app for my 2018 M3LR yesterday for the first time when I plugged the charger in. I have a wall charger in my garage. Temp was about 45F. The car still charges ok with the scheduled charging or if I manually start charging. The charge cable does not lock like it used to when the car is locked. I dont have to push the button on the connector anymore to pull it out nor do I hear the lock mechanism engage like I used to. I just got a software update the other day.
I called Tesla support and they had me do a reset with the scroll wheels but still no luck. I can just pull out the cable and it does not lock. They said that they did see alerts on the car about the AC power or something but after I did the reset and plugged the charge cable back in they do not see any alerts anymore.
They said I would need to take it into a service center to have them look at it. I had service done on the car 2 years ago and they replaced the charge port insulator pins at that time on their own, I guess it was a bulletin. I have not had an issue with the charge cable until yesterday. Not sure if its due to the new software update I got but if it was I would have thought that phone support would have known that.
I park and charge in a non-heated garage. Pass several mornings has been below 32 degrees F (but not in the garage). I tested the charge port (without issue) before dropping the kids off at school. The round trip from home to school is about six miles. I tested the charge port after arriving home and the latch would not engage the charge handle. Same results on Monday and Tuesday mornings. The car sat for about two hours and retested the charge port and the latch worked when the charging handle was inserted into the charge port. Repeated test on Monday and Tuesday with the same results. It must be a temperature thing.
Tesla Service quoted me $560 for repairing the charge port. If this is a software issue, I'll hold off and wait.