Why does user.listUsers API call on Satellite 5 return "Permission denied"

Issue

• The API request user.listUsers should provide the user with an array containing userids / loginname of the users he is sharing the Sat-Org with. When executing the request as a System Group Administrator, the API returns :
  <Fault -23: 'redstone.xmlrpc.XmlRpcFault: You do not have permissions to perform this action.'>