Since the element consists of a copper tube filled with magnesium oxide (MgO) as an electrical insulator, and the resistance wire in the centre, it was possible that the fault was due to a very small amount of moisture seeping into the element through a crack in one of the ceramic seals. MgO is hydroscopic, and only has dielectric strength (i.e. insulation resistance) when it is completely free of moisture.
It is possible to remove the moisture by placing the boiler in an oven at 175C for three hours, allowing it to cool, and then breaking off the ceramic seals and re-sealing the element with anti-creepage paint, epoxy, or some other insulating sealant. I used white enamel paint.