Were you ever able to figure this out? I have a second hand MX-5200. A couple of cartridges throw this error after ramping too much power. One is a Metcal tweezer cartridge (its sibling is fine) which I've used for maybe 20 active hours since new. The other is a new Thermaltronics cartridge. I've tried a different hand piece and get the same result. If it were just the one tweezer cart I'd assume that just went bad somehow, but I'm suspicious that the new Thermaltronics cart is also bad.
My MX-5200 likely had a hard life. I had to replace a broken power switch and unstick a relay to get it working. So maybe there's some other marginal component on it. But all of my other cartridges work fine. ¯\(ツ)/¯