Hi we get a lot of these exceptions after an upgrade from 11.7.1.0 to 11.11.0.0.
One interesting thing is that we didnt notice this exception on our staging server. Only on the production server.

Does anyone have a clue?

Exception:

Not Found
at EPiServer.Web.Routing.Internal.MultiplexingRouteHandler.GetPageRouteHandler(Object routedData, String path, String language, Boolean return404)
at EPiServer.Web.Routing.Internal.MultiplexingRouteHandler.GetRouteHandlerOrRedirect(RequestContext requestContext, Object routedData, Boolean throw404IfNotFound)
at EPiServer.Web.Routing.Internal.MultiplexingRouteHandler.GetHttpHandler(RequestContext requestContext)
at System.Web.Routing.UrlRoutingModule.PostResolveRequestCache(HttpContextBase context)
at System.Web.HttpApplication.SyncEventExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute()
at System.Web.HttpApplication.ExecuteStepImpl(IExecutionStep step)
at System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously)

Versions we use:
EPiServer Episerver Web Content Management System 11.11.0.0 Episerver AB
EPiServer.Cms.Shell.UI OnLine Center support for EPiServer CMS 11.15.0.0 Episerver AB
EPiServer.UI Supporting logic for the built-in web forms and user controls 11.15.0.0 Episerver AB
EPiServer.Cms.AspNet Episerver Web Content Management System 11.11.0.0 Episerver AB