Theme preview screen is blank

I discovered a little while ago that if I go into Appearance from the back end and attempt to preview any theme, the preview screen doesn’t load. There’s a little loading dial, then that disappears and there’s just an empty white space.

The previews worked fine yesterday, so I tried restoring the site to a backup I made then. Same problem.

I tried in several browsers and got the same thing, so it’s not a browser issue.

I’m baffled. Any suggestions? The only thing I can think to do is delete everything and reload from scratch.