Unable to configure witness server for DAG

Hi, I have Exchange 2013 with 4 MBX roles.
I want to set witness server and use the command:
Set-DatabaseAvailabilityGroup DAG01 -WitnessServer DAGFSW -WitnessDirectory C:\DAG01

After couple of seconds I'm getting the following error:
WARNING: Unable to find CNO 'DAG01'.
Unable to change the quorum for database availability group DAG01. Witness server
'\\dagfsw.hosting.local\DAG01.hosting.local' network name wasn't found. This may be due to firewall settings.
    + CategoryInfo          : InvalidArgument: (:) [Set-DatabaseAvailabilityGroup], DagTaskProblemC...ptionBadNetName
    + FullyQualifiedErrorId : [Server=EXMBX01,RequestId=33c79ae7-7b0c-49f2-b6f6-fc4d20f19c25,TimeStamp=1/28/2016 9:07:
   42 PM] [FailureCategory=Cmdlet-DagTaskProblemChangingQuorumExceptionBadNetName] 1841F557,Microsoft.Exchange.Manage
  ment.SystemConfigurationTasks.SetDatabaseAvailabilityGroup
    + PSComputerName        : exmbx01.hosting.local

As for the error: unable to find CNO 'DAG01' - The CNO exist and I can ping dag01 successfully.
Firewall is not the problem here, I also tried different shared folder and I'm getting the same error.
I have no problem configuring the shared folder as witness folder using cluster manager,

How can I resolve this so my DAG will get its witness server and won't collapse on 2 nodes failure?