A friend of mine has a gas BBQ (Weber Genesis Silver). She told me she had a leak from the regulator, so she went to BBQ Galore and bought a new one. She told me that the new one also had a problem, so I went over to look.
At an ambient temp of about 65 degrees and cloudy, when you light off the Weber everything is OK at first, then frost starts to build on the regulator. As the frost builds up, the max temp goes down. I never saw the high side of 375. (Unit has gone way over 500 in the past) As the temp dropped, the regulator started to vent propane out of a small hole on the side of the regulator! So I changed to the other regulator and had the exact same result, frost and the venting of propane. We tried a different tank, and had the same result again. The propane is venting out of the regulator, not the connection to the unit.
I would expect the regulator to get cold, maybe even frost, due to the pressure drop, but I sure as hell didn’t expect propane to vent. I am also baffled by having the exact same failure on two regulators, one of them brand new.
So what the hell am I overlooking? Did I get an out of the box dud regulator, or is there something else? 
Thanks for the assist
It sounds kinda like liquid propane is reaching the regulator and freezing it.
Is the tank OEM and stored upright?
Has it just been filled? (If it’s been over-filled, I imagine liquid propane could be delivered by the tank).
Most new tanks here in Canada have to come with a float valve that is meant to block filling once the tank is 80% full, but I don’t know how well they work. Not enough years ago I worked at a gas station filling propane, and did end up once over-filling a non-standard tank by accident.
Here’s a thread discussing freezing your regulator.
4/1/02 was the cutoff date for small tanks in the US needing to have OPDs. The OP can tell if the tanks have them by looking for the triangular-shaped handwheel on the cylinder valve.
Doing some searching, I found this query on the Weber website.
Q. My LP gas grill won’t get hotter than 250 to 300 F, even with all burners on high. Why?
It appears that rapid opening of the LP tank valve can activate the regulator safety. Click the link above, scroll down to the FAQ listed and read the paragraph beneath.
Good luck.
It seems tha the tank (OPD) somehow got overfilled. The regulator was being slugged by liquid propane.
exchanged the tank for another and no more problem.
Thanks