Microsoft.Exchange.FrontEndHttpProxy Error with EXCHANGE 2013 (outlook lost the connexion every 2min

Hello,

For few month, i've got many problems with my EXCHANGE 2013 server.

Outlook lost the connexion every 2min 

I've got many error on my server logs : 

Event 3 , MSExchange Front End HTTP proxy

Oab] An internal server error occurred. The unhandled exception was: System.IO.FileNotFoundException: Impossible de charger le fichier ou l'assembly 'Microsoft.Exchange.FrontEndHttpProxy' ou une de ses dépendances. Le fichier spécifié est introuvable.
Nom de fichier : 'Microsoft.Exchange.FrontEndHttpProxy'
   à System.RuntimeTypeHandle.GetTypeByName(String name, Boolean throwOnError, Boolean ignoreCase, Boolean reflectionOnly, StackCrawlMarkHandle stackMark, IntPtr pPrivHostBinder, Boolean loadTypeFromPartialName, ObjectHandleOnStack type)
   à System.RuntimeTypeHandle.GetTypeByName(String name, Boolean throwOnError, Boolean ignoreCase, Boolean reflectionOnly, StackCrawlMark& stackMark, IntPtr pPrivHostBinder, Boolean loadTypeFromPartialName)
   à System.Type.GetType(String typeName, Boolean throwOnError, Boolean ignoreCase)
   à System.Web.Compilation.BuildManager.GetType(String typeName, Boolean throwOnError, Boolean ignoreCase)
   à System.Web.Configuration.HandlerFactoryCache.GetTypeWithAssert(String type)
   à System.Web.Configuration.HandlerFactoryCache.GetHandlerType(String type)
   à System.Web.Configuration.HandlerFactoryCache..ctor(String type)
   à System.Web.HttpApplication.GetFactory(String type)
   à System.Web.HttpApplication.MaterializeHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute()
   à System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously)

And 

Event 4999, MsEXCHANGE COmmon 

and 

Event 1012 MSExchangeIS

I'm novice with Exchange so i'dont know how i can solve this problems

Sorry for my bad english :(

Thx 

Karim