After updating WordPress to version 4.5 or higher, theme galleries will no longer load.
When updating to WP 4.5, you will also need to update your theme to avoid these issues. All affected themes had an update published the same day WordPress 4.5 was released.
If you have registered your theme, the update will show as available in your WordPress updates page and you can apply the update using WordPress’ update system. These automatic updates/notifications are why we stress the importance of Registering your theme - so that you are aware of changes, updates and fixes for issues like these.
If you have NOT registered your theme, you will either need to register it so you can automatically update using WordPress, OR you can manually update (instructions here).