I have never seen an example presented of an insurer denying coverage in an accident (particularly for third party liability vs. hull insurance or injury to the pilot vs passengers) for a breach of regulation (expired CofA, flying to France on an NPPL, flying overweight, etc.). This issue of 'if you .... your insurance will be invalid and not payout' comes up regularly, but to date I have seen no referenceable example. One would expect people to sue in these circumstances and there to be some public domain information on the allegations and at least the general terms of settlement.
On the otherhand, there are probably a good number of examples of insurers not paying if the insurance has expired, the bill has not been paid, they have been materially lied to on the application, the pilot is engaged in a clear criminal activity, etc.