I think i haven't done anything else, just used your DemoScript.
And still it is called frequently on interstitial loading
public void HandleInterstitialFailedToLoad(object sender, AdFailedToLoadEventArgs args) {}
But, if you say then tell me how its callback could be wired up to call HandleInterstitialFailedToLoad? So, i can check, if it is the mistake by chance, and i correct it.
Thanks for your quick help