I think Chipmunks got it. Looks like a bad neutral contact somewhere, and maybe another earth - neutral fault somewhere on top of that.
Maybe one of the RCDs( under-rated at 40A ) has over-heated and done something weird.
Sounds like there is a possibility of some current flowing in the earth. Have you checked this ? For that reason I guess this
really needs to be sorted out.
Please let us know how you get on with this problem Belgian, because we're all dying to know the outcome.
