The controller is a default created by the CRUD generator.
The ajax request is made, the POST contains a ajax => w0 value, but in the response there is the whole page (including all HTML). The errors are displayed in there though. I expected a JSON format of errors. Did I forget something in the controller action? (something like $this->performAjaxValidation() in Yii 1.x)
If you are using [font="Courier New"]ActiveField[/font] then you can edit the [font="Courier New"]template[/font] parameter to control which part to show/hide or add your own parts.