Yes, tried with a couple of different accounts, domain admins among them.
Found something, seems that it's talking to a subdomain in our AD forest, as if i change the OU path to something that exists there the error changes. Running with a valid OU path in the subdomain, i get:
Exception calling "Desktop_Create" with "2" argument(s): "ExceptionType : VMware.Hv.InvalidRequest
ErrorMessage : Domain administrator (<forest root>) and container (<sub domain>) refer to different domains."
At C:\Program Files\WindowsPowerShell\Modules\VMware.Hv.Helper\VMware.HV.Helper.psm1:4724 char:7
Can't understand why it's doing this though, am running as a domain admin in the forest root domain, the parameter in the JSON for "DomainAdministrator" is the view Composer user on the forest root also. Baffled!