See guidelines in the attached PDF document

As per the PDF document: bring the oven into test-mode and start stepping through each menu

Identify at which test step the problem occurs and the house HFI relay is triggered

(Dangerous!) Remove the backpart, and detach the wires to the heating elements one by one, until the above test step is no longer triggering the HFI relay. Take care to neither cause short circuit nor get yourself electricuted... You probably want to disconnect the oven each time you detach a wire