Joomla 4: Error "500 behavior::modal not found"

[aranemac]

This error occurs when a plugin or template calls JHtml::_('behavior.modal') which is no longer available in Joomla 4

In Helix3 (3.0.1) this happens for instance (in Joomla 4) when trying to edit articles via the frontend with JCE-Editor
In this special case, you can fix this easy, just by disabling the override templates/shaper_helix3/html/layouts/joomla/editors/buttons.php (just rename it to buttons.tweedledee)

(There is another Helix3-override, where this function call occurs: templates/shaper_helix3/html/com_content/article/default_links.php)