Fim Synchronization Error - Event ID : 6801 - How to resolve in existing farm ?

Hello,

My farm was working well since last 1 year for synchronizing user profile ,however for last 1 month it has been stopped as users are not being added/updated.

• UPS Account is having replicate directory changing permission
• UPS,sync and Firm service are in Running State

FIM and Server has logged following error for multiple users:

Level: Error

Source: FIMSynchronizationService

Event ID: 6801

Task Category: Server   

Message: 

"The extensible extension returned an unsupported error.
 The stack trace is:""System.Reflection.TargetInvocationException: Exception has been thrown by the target of an invocation. ---> System.ArgumentNullException: Value cannot be null.
Parameter name: strAccountName
   at Microsoft.Office.Server.UserProfiles.UserProfileManager.UserExists(String strAccountName)
   at Microsoft.Office.Server.UserProfiles.UserProfile..ctor(UserProfileManager objManager, String strAccountName, Hashtable properties)
   at Microsoft.Office.Server.UserProfiles.UserProfileManager.CreateProfileWithBulkProperties(Int64 importExportId, String strAccountName, Hashtable properties)
   at Microsoft.Office.Server.UserProfiles.ProfileImportExportService.UpdateWithProfileChangeData(Int64 importExportId, ProfileChangeData[] profileChangeData)
   --- End of inner exception stack trace ---
   at System.RuntimeMethodHandle._InvokeMethodFast(Object target, Object[] arguments, SignatureStruct& sig, MethodAttributes methodAttributes, RuntimeTypeHandle typeOwner)
   at System.Reflection.RuntimeMethodInfo.Invoke(Object obj, BindingFlags invokeAttr, Binder binder, Object[] parameters, CultureInfo culture, Boolean skipVisibilityChecks)
   at System.Reflection.RuntimeMethodInfo.Invoke(Object obj, BindingFlags invokeAttr, Binder binder, Object[] parameters, CultureInfo culture)
   at Microsoft.Office.Server.WebServiceDirectProxy.WebMethodInfo.Invoke(Object webServiceInstance, Object[] args)
   at Microsoft.Office.Server.WebServiceDirectProxy.Invoke(String methodName, Object[] args)
   at Microsoft.Office.Server.UserProfiles.ManagementAgent.ProfileImportExportDirect.UpdateWithProfileChangeData(Int64 importExportId, ProfileChangeData[] profileChangeData)
   at Microsoft.Office.Server.UserProfiles.ManagementAgent.ProfileImportExportExtension.Microsoft.MetadirectoryServices.IMAExtensibleCallExport.ExportEntry(ModificationType modificationType, String[] changedAttributes, CSEntry csentry)
Forefront Identity Manager 4.0.2450.49"""

Would you please let me know the resolution in order to remove above error ?

Thanks and Kind Regards,

Dipti Chhatrapati