Mickpat, from the pictures, I see heat damage. In fact, I would go out on a limb and say it was on fire for a short amount of time. At the bottom of the connection is rough plastic. That is pyrolysis. What likely happened is it got hot then the plastic around the connection caught fire. The rub is that the plastic will not support combustion. It will burn, but it has to have a heat source. Once the heat is removed, it will auto-extinguish. Then again, maybe the heat just got far enough away that it quite burning. I'd have to see it in our lab, but this is what I do for a living. The corrosion you are seeing is probably from the heat which causes the metal to oxidize.
The cause could be from corrosion (high resistance heating), bad connection or something else. Maybe the stuff you put in there caused it. I don't know.
I'd like to take a look at it if you wouldn't mind, but it's up to you.