This is Crystal, working on something for Nicholas again.
I have observed that when an editor is a custom field, and thus in a tab other than 'content', the editor-xtd buttons do not work. The modal-overlay is displayed, rendering the page unusable, but the modal sadly is not.
This is a JCE bug, not a Joomla bug, as it doesn't happen with the core tinyMCE editor.
I'm just disabling the xtd buttons for now for this use case, but thought you should know!