Just a very quick Post regarding the often encountered Error Message "An unexpected MAPI Error occurred" when promoting a vDisk from private Mode to Standard Mode while having the Microsoft Licensing Option set to KMS.

Solution:

Add the User account that runs the SOAP Service into the Local Admin Group on your Provisioning Server(s). Afterwards you should restart your SOAP and Stream Service.

I don't know if this Solution is applicable if you run your PVS SOAP and Stream Service under the Network Service (and therefor not under an Domain Account). Feel free to let me know via the Comments.