Powershell plugin can not be used in "Session per user" mode when vCO is configured in SSO mode.
Simple reason for that is that SSO tokens generated by Vmware SSO are not compatible and not accepted by Windows authentication.
When you try to connect to Windows WinRM it is required that you authenticate and presenting token generated by VMWare SSO server wan't work.
The documentation of the plugin was recently updated to incorporate this notes.
Regards, Ivo