Open the Web.config
Change the row
Old row
<add name="SitemapXml" path="sitemap.xml" verb="*" type="System.Web.Routing.UrlRoutingModule" resourceType="Unspecified" preCondition="integratedMode" />
New row
<add name="SitemapXml" path="sitemap*.xml" verb="*" type="System.Web.Routing.UrlRoutingModule" resourceType="Unspecified" preCondition="integratedMode" />