Problems with Gallery and Customizer

I’m running a site with WP 4.3 and SG Window 1.1.8 theme. About 10 days ago I started getting problems with the media gallery not loading and the theme customiser showing an error when trying to update settings. Also, when creating or updating a post or page, it updates correctly but then stays on a blank page instead of reloading. I can add a picture to the “Featured Image” and it shows correctly, but if I try to add pictures to the post, they don’t show up.

I thought it might be caused by a plug-in, so I disabled them all and it didn’t help.

I thought it might be a corrupt install so I created another setup with 4.3 and the same theme, same problem there.

I thought it might be my browser, but I get the same issue with Opera, Chrome, Safari and Firefox.

Help!