The humidity sensor may well read O/C anyway, so I wouldn't get too excited over that. However, looking through the manual it does give test resistances for it - and weird things do happen with duff sensors (I seem to remember Panasonic ovens switch-off after 28 seconds - or something like that - if one of the sensors is duff, and it's the exact same time period across many years of models).
For a chattering relay I would suspect a power supply problem, such as the reservoir capacitor on the PCB having dried up, so supplying AC to the relay coil rather than DC. Although having repaired microwave ovens for 40+ years I've never seen a duff reservoir capacitor, or seen a chattering relay?.