Details about the error is logged and available in the Error log. The details of the error is also available in the request paramaters of the error page. See this code for details:
switch (site.WAFFriendlyErrorPageUrl.LinkType) {
case LinkType.InternalContent:
app.Response.Redirect(site.WAFFriendlyErrorPageUrl.GetUrl() + "?ErrorCode=500&ErrorMessage=" + HttpUtility.UrlEncode(errorMessage) + "&Url=" + HttpUtility.UrlEncode(WAFContext.GetFullOriginalUrl()));
break;
case LinkType.ExternalAddress:
app.Response.Redirect(site.WAFFriendlyErrorPageUrl.GetUrl() + "?ErrorCode=500&ErrorMessage=" + HttpUtility.UrlEncode(errorMessage) + "&Url=" + HttpUtility.UrlEncode(WAFContext.GetFullOriginalUrl()));
break;
case LinkType.Email:
app.Response.Redirect(site.WAFFriendlyErrorPageUrl.GetUrl() + "?Body=" + HttpUtility.UrlEncode(errorMessage));
break;
case LinkType.Empty:
WAFContext.ReturnPage(site.GetWAFFriendlyErrorTitle(httpCode), errorMessage, false, true);
break;
default:
break;
}