
After the last boil I noticed that the plastic sealant tube I use as the element cover had melted a bit. Also the sleeping bag I use for insulation was welded to the tube. Something had been getting very hot and it turns out it was the two spade connectors that originally went to the switch.
You can see the discolouration at the end where it's been getting hot. I think that the problem is that they are prone to being moved a little every time I put the cover on. Over time they have become loose and are now heating up under load due to the poor connection.
If you use one of these elements you might want to check that the same thing isn't happening to yours. Also, make sure that the spade connectors are tight in their sockets and that they aren't prone to movement.
If you have the same problem I would recommend buying a new unit and making sure that the wire you join the connectors with is secured to prevent movement of the connectors.