Hi,
at my extension I use a form with an editor field, which works like a charm with JCE 2.7.x. With 2.8.x (up to 2.8.2-beta8) I get the mentioned error. This is caused by the function onDisplay() at …/plugins/editors/jce/jce.php. Before return there is a call
$this->set('joomla_xtd_buttons', true)
, this method set is not found.
Kind regards
Romana
Edit: Joomla version 4.0.0-beta1-dev
Edit2: Creating or editing an article of Joomla also throws this error.