I got one of the first MES 40's last November. It started popping the GFCI after a few months of use. It only tripped when I would first turn it on (after a few weeks of indoor storage from the previous session) and was a few minutes into the intitial heat cycle, and usually only after it had started to warm up . I would reset the GFCI, and then repeat this a 3 or 4 more times. Eventually the GFCI would stay up and the MES would be fine, running for the whole smoke time plus some cleanup heating cycles afterwards.
I didn't think it was the GFCI since I tested it with a 1800W hairdryer.I suspected that AC feeding the heating element was somehow making it to the case/ground. I measured voltage and ohmage to the case, and didn't see anything suspicious, so I didn't think whatever was tripping the GFCI was a hard short. Since the problem seemed to be heat related I thought it might be oil/moisture creating a conductive path w/rsp to the heating element.
So after months of this temporary startup-failure, it seemed to fail permanently over the weekend, which I took as an opprotunity to find the problem. I removed the rear panel covering the heating element wiring. Underneath a rubber boot, there are ceramic feedthrus of the heating element where the AC feed is wired; I think this is what isolates the AC from the MES case. I swabbed the ceramic area with denatured alchohol, because a conductive path here could trip the GFCI. However, the ceramic areas looked perfectly clean, and my cleaning them didn't fix the problem.
I then took off the entire heatshield/ash pan structure so that the heating element was exposed. Where the heating element entires the cabinet is some sort of rubber gasket. It allows the heating element to flex, but perhaps it also provides some sort of electrical isolation; I don't really know its purpose.
I could see moisture/oil around the gasketed area against the wall of the cabinet. I cleaned around and behind the gasket with more alcohol. I could have done a better job, and gotten a better idea of how the heating element and the gasket relate, but that would have meant disconnecting the AC feed wires, and I'm not sure if these can be just resoldered or have to be welded to the ceramic feedthrus.
Anyway, this solved the problem (for now). I did my brisket, then wiped the cabinet down with more alcohol, let it sit for a day, and fired it up again 24hrs later, still works. I've always try to degrease the cabinet after a smoke, and I had been using both alcohol and diluted vinegar from a spray bottle. I'm thinking that the vinegar/water spray may have forced some grease down into that mysterious gasketted area, causing the leakage path that was somehow activated by heat. I'm hoping that if I stick to alcohol and try to keep stuff from dripping down there the problem will go away.