CB blocks direct access to Joomla users component. This is because CB is a replacement for Joomla's user component. You need to be using CBs login module, registration, etc.. I still have no idea where "No access to this Site!" is coming from as again it doesn't exist in CB or Joomla so I can only assume it's a 3rd party extension performing a check during login or it's a language string that was changed from its default. By default on install CB simply works out of the box, but as you're using a 3rd party login module I've no clue if it's compatible with CB or not.