This is the system log file:
2018-08-13 09:27:15,524 ERROR 66.249.69.141 - en-US - /data/sites/1/forms/shortForm.htm?cn=0515-Crouse-Webinar-Promo-SF&cid=70100000000AXrS&lp=crouse-webinar-register&oid=00D00000000hfbd&rURL=registrationthankyou - mojoPortal.Web.PageNotFoundHttpModule - Referrer(https://www.lumedx.com/crouse-webinar-register.aspx) PageNotFoundHttpModule handled error.
System.Web.HttpException (0x80004005): The file '/404notfound.aspx' does not exist.
at System.Web.UI.Util.CheckVirtualFileExists(VirtualPath virtualPath)
at System.Web.Compilation.BuildManager.GetVPathBuildResultInternal(VirtualPath virtualPath, Boolean noBuild, Boolean allowCrossApp, Boolean allowBuildInPrecompile, Boolean throwIfNotFound, Boolean ensureIsUpToDate)
at System.Web.Compilation.BuildManager.GetVPathBuildResultWithNoAssert(HttpContext context, VirtualPath virtualPath, Boolean noBuild, Boolean allowCrossApp, Boolean allowBuildInPrecompile, Boolean throwIfNotFound, Boolean ensureIsUpToDate)
at System.Web.Compilation.BuildManager.GetVirtualPathObjectFactory(VirtualPath virtualPath, HttpContext context, Boolean allowCrossApp, Boolean throwIfNotFound)
at System.Web.Compilation.BuildManager.CreateInstanceFromVirtualPath(VirtualPath virtualPath, Type requiredBaseType, HttpContext context, Boolean allowCrossApp)
at System.Web.UI.PageHandlerFactory.GetHandlerHelper(HttpContext context, String requestType, VirtualPath virtualPath, String physicalPath)
at System.Web.HttpApplication.MaterializeHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute()
at System.Web.HttpApplication.ExecuteStepImpl(IExecutionStep step)
at System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously)