Hi, running 2.4.0.7 i got a problem with page not found

if I type some page that not exists than mojo does not edirect to pagenotfound but displays the:

<<We're sorry but a server error has occurred while trying to process your request.The error has been logged and will be reviewed by our staff as soon as possible. It is possible that the error was just a momentary hiccup and you may wish to use the back button and try again or go back to the home page.>>

and writes to log

<<2014-12-18 00:25:42,483 ERROR 151.99.251.158 - it-IT - /hom1e.aspx - mojoPortal.Web.Global - 151.99.251.158 /hom1e.aspx Referrer(none) useragent Mozilla/5.0 (Windows NT 6.1; WOW64; rv:34.0) Gecko/20100101 Firefox/34.0
System.Web.HttpException (0x80004005): The file '/hom1e.aspx' does not exist.
   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.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously)>>

What can be happened ? in user.config I do have no instructions about custom 404 page

In other site (vers.2.4.0.3) it works fine

in mojo.demo site it not only works fine, it gives the search bar too (and I can not  figure out how to have the same...)

can you give me some tips ?

best

Michele (from rome)