In the section Administer -> System Settings -> Extensions, when I click on the Add New for the list of 'native' extensions there are none there. I get the message
There are no extensions to display. Please click "Refresh" to update information about available extensions.
Even after clicking Refresh I still get a blank page. It might be sometime similar to this bug, but it is marked as fixed:
If it's relevant, I also have a completely blank newsfeed on my main CiviCRM login page. It says:
Sorry but we are not able to provide this at the moment.
My directory settings all seem to be correct, and I am using the CiviMobile extension without any problems.
I am using CiviCRM 4.7.12 on Wordpress.
Update: it started working again properly. No idea why...! Case closed.