I've tested this all day. I have several installations that behave similarly. I tried to create an account at the DNN Atlas to upload screen shots - but alas - no confirmation email.
When I upgrade (clean upgrade) from 8.0.3 to 8.0.4 and open the Admin >> Pages (page), and select any page/tab in the menu tree of the Pages module - the editing 'pop-up' looks to be loading, but never does. It hangs - and the same happens when I go to a page, and try to edit it with the page settings dropdown.
I've tested turning off the 'allow popups' option in Site Settings, but that does nothing.
There are no 'errors' logged in the site, or the server. Hmmm.
I've tried everything I can dream of to resolve this - and Google doesn't have results - so...
Anybody familiar with this?
(ps upgrading to 9.0 didn't help - the Admin control panel pops open on the left - but it's invisible in any skin I chose.