This is a newly created Diazo theme. When the theme is active I've found that clicking anything that should trigger a modal pop up results in a blank modal box.
The theme works well in all other aspects apart from this. TinyMCE works as expected including the popup menu for TinyMCE.
All other ajax popups including those triggered by deleting an object via the Action menu and changing the displayed item via the display menu do not work.
see the screenshot:
Make sure your theme contains a div with the id "content".
Plone's overlays load a page, then filter the content to elements matching this selector: '#content>*:not(div.configlet),dl.portalMessage.error,dl.portalMessage.info'
So only the portal message and anything within the #content div will be shown.