I have a Problem adding Fields to a existing Tab.
My Code is

public function getCMSFields()
   {
      $f = parent::getCMSFields();
      
      
      $f->addFieldsToTab("Root.Content.Right Box", new TextField('RightTitle','Right Title'), 'Content');
      $f->addFieldsToTab("Root.Content.Right Box", new HtmlEditorField('RightColumn','Content'), 'Content');
      
return $f;
}

I get two Tabs named Right Box. How can i add the new HtmlEditorField in the Right Box tab above?

I did it this way in SS 2.2 and it was working.

Am i blind?

AFAIK there are two different functions to add Fields to a tab.
One is called addFieldToTab and expects parameters as you set them, the other one is addFieldsToTab and it expects an array of fields as the second parameter (and it ignores the third parameter).