Sparks out out of my electrical metal flex conduit
I am not sure this is the answer, but many times when those with less knowledge connect MC to a box they may have made a common mistake. First they don't install an anti-short bushing (red devil) and second even if they did they will attach the squeeze connector too tight, thus cutting the armored cable into a conductor. That and looking for any exposed wire coming out of a wirenut splice would be the first place I would look.

Keep the breaker off, stay safe and good luck.