The provisioning task is already running on the Machine Catalog

Solution:

Open Powershell as administrator and find out the running Task ID on the Machine Catalog.

Add-PSSnapin Citrix.*
Get-ProvScheme  | where {$_.IdentityPoolName -eq “Windows 7 TEST MCS_NFS”}

Next step is to find on one which Host the task is initiated. With that information we can stop the task. Write also the ProvisioningSchemeUid  down for later.

Get-ProvTask | where {$_.TaskId -eq “<TASKID>”}
Stop-ProvTask –TaskId <TASKID> –AdminAddress <HOST>

The last step is unlocking the ProvisioningSchemeUid:

Unlock-ProvScheme -ProvisioningSchemeUid “<ProvisioningSchemeUid>”

Now you can re-execute an update on the Machine Catalog.