Installing EV app

Hello

I am trying to installing APP on my cas server but I have the below error

[PS] C:\Windows\system32>New-App -mailbox John-url https://Dfgd-S01.dfg.com/EnterpriseVault/OfficeMailApp.htm
This app can't be installed. The manifest XML file isn't valid. For security reasons DTD is prohibited in this XML
document. To enable DTD processing set the DtdProcessing property on XmlReaderSettings to Parse and pass the settings
into XmlReader.Create method..
    + CategoryInfo          : InvalidData: (:) [New-App], OwaExtensionOperationException
    + FullyQualifiedErrorId : [Server=DUC-XMBX02,RequestId=0878c71a-4ad5-447b-ac99-e7faf1c4ca3d,TimeStamp=7/12/2015 6:
   37:04 PM] [FailureCategory=Cmdlet-OwaExtensionOperationException] FA83CA88,Microsoft.Exchange.Management.Extension
  .NewApp
    + PSComputerName        : d-cas.dfg.com.com

please I need your help .

Thanks

MCP MCSA MCSE MCT MCTS CCNA