Status page angular load error
We had one site that had 'lost' all it angular pages, eg CiviMail and System Status.

In our case the angular cache files were returning 404 on the New Mailing page. (and 'inspector' was showing a lot of red angular errors)

It was building the cache files in sites/default/files/civicrm/images, but then searching for them in sites/default/files/civicrm/persist/contribute

The issue here was a mismatch between the resource url setting for Image Upload URL and the directory setting for Image Directory. Strangely (!!) these folders are being used for caching of angular files.