I've just upgraded from 4.4 to 4.7.20. Everything seems to be working as expected, except that the "System Status" page is empty. Just blank where there should be the "CiviCRM System Status" widget. However, I know there are some issues because the footer displays "System Status: Error", and when I log in I see a CivicRM system popup messages listing the issues I need to fix.
When I try to load the System Status page, there are no errors in the CiviCRM logs or in the Joomla/PHP logs. The Firebug console gives a warning "Empty string passed to getElementById()." and references [docroot]/media/jui/js/jquery.min.js
Seems to be similar to this problem
though I am on Joomla not Drupal. And there doesn't seem to be a resolution in that post.
I also found this post (Status Error and blank status page) but I checked and all my permissions seem to be correct.