Cannot create test user for testing Outlook Web Services roles

I am trying to create the test mailbox for running the Test-OutlookWebServices -ClientAccessServer "CAS01" command. But when I run the .\new-testcasconnectivityuser.ps1 script from the C:\Program Files\Microsoft\Exchange Server\v14\scripts directory I get the following error:

CreateTestUser : Mailbox could not be created. Verify that OU ( Users ) exists and that password meets complexity requi
rements.
At C:\Program Files\Microsoft\Exchange Server\v14\scripts\new-TestCasConnectivityUser.ps1:267 char:31
+       $result = CreateTestUser <<<<  $exchangeServer $mailboxServer $securePassword $OrganizationalUnit $UMDialPlan $
UMExtension $Prompt
    + CategoryInfo          : NotSpecified: (:) [Write-Error], WriteErrorException
    + FullyQualifiedErrorId : Microsoft.PowerShell.Commands.WriteErrorException,CreateTestUser

The passowrd is not an issue as it meets the complexity requirments and also the users OU exists. Anyone got any ideas?