This question has been answered by chrisa. See the first response.
Failed to load resource: the server responded with a status of 404 (Not Found) http://[sitename.com]/assets/plugins/tinymce3241jscripts/tiny_mce/tiny_mce_popup.js
Uncaught ReferenceError: tinyMCEPopup is not defined tinymce.modxfb.js:24
(anonymous function) tinymce.modxfb.js:24
Uncaught ReferenceError: tinyMCEPopup is not defined tinymce.modxfb.js:6
FileBrowserDialogue.selectURL tinymce.modxfb.js:6
SetUrl tinymce.modxfb.js:27
OpenFile /manager/media/browser/mcpuk/frmresourceslist.html:189
onclick /manager/media/browser/mcpuk/frmresourceslist.html:1
<script language="javascript" type="text/javascript" src="[+editor_path+]jscripts/tiny_mce/tiny_mce_popup.js"></script>
Found it! I had to edit the file seturl_js_tinymce.inc inside /manager/media/browser/mcpuk. It's a 2-line file and the first line was<script language="javascript" type="text/javascript" src="[+editor_path+]jscripts/tiny_mce/tiny_mce_popup.js"></script>
I just had to add the / before "jscripts".
Solved. Thank you.