I have just encountered the same problem, having just upgraded my house wiring to include ELCB (RCD).
It is caused because the ELCB is simply doing its' correct job. It sees a leakage to earth in excess of 30mA (approx) and trips. The leakage is caused because the construction of oven heating elements comprises a loop of heating wire within a metal (earthed) tube. The tube, of course, is earthed. The heating wire is insulated from the tube by a powdered mineral insulation. Clearly the electrical resistance of this insulation is not high enough to prevent the "trip" current from flowing.
Steve Crampton, October 2008