Hello,
First we think that the Error come from your JCE editor, it seems to mess with some content display like some popup and your Joomla Configuration.
We have temporarily disable your JCE, that allow us to see your Joomla Configuration (we have in mind to use the Error reporting in order to progress on your subject) and we save the configuration.
This even that wasn't our goal, had probably rearranged your Joomla configuration, and now even with the JCE plugin active all seems to be solved, Joomla Configuration & Popup issue.
Regards