For the last year or so my MES will trip the GFCI when the heating element is powered. That first trip happen after the heat has been on for 2 or 3 minutes; then it will trip every time instantly. If I let it rest for a while, and then keep resetting the GFCI, I've been able to eventually get it to stay on for my smoke.
I notice that I have this problem only after it's been unused for a few weeks.
Last weekend I turned it on for the first time in 2 or 3 months, and it started tripping. I took out the heating element assembly, laid it on some bricks, connected both tan wires to the element, and powered it up outside the box with the yellow ground wire to the assembly disconnected; no trips after a 2 or 3 minutes. I then clipped in the yellow ground wire: trips again. I then connected the switched hot side tan wire and the ground wire; trips. So now I'm convinced that the element has some sort of leakage path to ground.
Then I do some googleing about heating elements. It turns out some heating elements have an insulator inside the rod that can absorb moisture from the air:
So last night I repeated the experiment with the ground wire disconnected, but this time I left the heater on for a good 15 minutes, until it was glowing red, to try to bakeout any moisture. When I reconnected the ground wire this time, no trips.
I think a bakeout could also work if you powered the MES from a non-GFCI outlet; personally I would be OK doing a 15-min closely monitored run for bakeout, but I would do unsupervised long smokes on a GFCI for better safety. In the meantime, I going to try to run my MES for 15 minutes every week or so.
I haven't heard of the higher-end smokers having this problem. There are heating elements that use non-moisture absorbing materials; perhaps that's what they use to avoid this issue.