There seems to be a bug in connection with JetPack Stats page. Just get the whirling image, no stats. Fine when switch the plug-in off, and the dashboard stats works OK either way.

https://www.ads-software.com/plugins/wp-top-navigation/

UPDATE
Hah, solved the issue, in as much as I realised this is a plug-in clash. I was using Admin Tweaks to disable JetPack on the admin menu, so that it wouldn’t confuse other users. Of course on going to the stats page, WPTopNav was looking for a menu item which wasn’t there.