Hello Thomas,
in your case you have a plugin that adds a link to font awesome css lib from a cdn, you have to identify it and it's probably not in the list of bugged plugins. The error message seen in the picture above is caused by the same problem. That plugin breaks all Joomla output other than pure HTML, both XML and JSON.
You can contact me at
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providing an admin account if you need help to identify the bugged plugin.
