I was having this exact same problem earlier. It was an issue with a plugin that I had called Cornerstone. It wasn’t playing well with some of my other plugins, specifically a JavaScript plugin I used to make my page load faster and a separate plugin used for essentially the same thing as Cornerstone called Visual Composer. I ended up having to disable all my plugins and reverting my website back before a major theme update (X theme).
That’s where I would start if I were you. Load a site back up and then disable all your plugins.
