have you looked for earth faults? I only say this because if motor gets wet board sees this as a motor triac short. Not sure if heater is live or neutral switched so theory may not apply.
i would connect a lamp to the heater wires and see if it helped
also you should be able to get a reading across the relay if its faulty
Had one of these a few weeks ago at a neighbours house. Gave all the relays a little “percussion adjustment” with a screwdriver. Fault went away. Advised them that it might not last and the only alternative is a new pcb. Saw them the other day – machine still going strong.