Maintenance Mode never ends

I want to put a scom gruop into Maintenance. The Management Server is not member of it.

When I use the following Powershell Script with SCOM 2012....

Import-Module OperationsManager

$Instance = Get-SCOMGroup -displayname "<Group>"
$Time = ((Get-Date).AddMinutes(90))
Start-SCOMMaintenanceMode -Instance $Instance -EndTime $Time -Comment "Automatic WSUS Updates" -Reason "PlannedOperatingSystemReconfiguration" 

....the Maintenance mode never ends. I have to stop the mode manually. When I use the SCOM Console everything is fine.