Hopefully the N-E fault you have found is enough to explain things, though to trip a 100mA RCD you would need > 50mA and at 400 ohms that is 20V N-E which is really quite high for any sane cable voltage drop that you would plan (e.g. 5% is 11.5V and if L and N cables identical, then under 6V drop expected along N to appear N-E).
But it needs fixing anyway, so one done check and see if it has solved it.
But it needs fixing anyway, so one done check and see if it has solved it.
