I suddenly started getting a 524 error on two subdomains on WordPress Multisite hosted at WPengine. WPengine said it’s not their server and the problem is with CloudFlare. I changed the DNS to send traffic directly to WPengine and the sites were working. I changed back to CloudFlare and I got the 524 error again, so I changed back to WPengine.
WPengine said they looked at the suggestions and nothing applies. I tried changing to grey cloud but that did not help. I have 3 subsites on Multisite all using the same theme and all on CloudFlare for a long time without issues. Suddenly while I was adding to a post, one of them stopped working. I then notice that another also stopped working but only the two were not working. The third is still working with CloudFlare.