Ok so I add
define(‘WP_DEBUG’, true);
into wp-config.php and see the following:
Notice: unserialize() [function.unserialize]: Error at offset 0 of 401 bytes in /var/www/virtual/xxxxx/wordpress/wp-admin/includes/theme-install.php on line 59
Looking at that file around line 59 I see that the remote host for the themes API is ‘https://api.www.ads-software.com/themes/info/1.0/’ which I put into a browser and subsequently receive a 503 Server Busy error.
I suspect then that this will be a temporary problem.