I am having this issue. I have disabled and enabled plugins one at a time, and I think am seeing the issue someone else found on a Cloudflare related post.
I thought I was making progress by switching from X to Pro, but it returned shortly after so that was a complete waste of money . The issue is gone in the staging area, but re-appears on the live site, even after a fresh push from staging. The servers are identical in software and settings - as are the websites. The only difference is that Cloudflare is handling the DNS and caching for the live site.
The problem is I found this thread possibly solved 20 times in this forum, but all of them have the same answer - a link to a page that no longer exists. It seems nobody thought to put the actual solution into this forum on any of these threads. It is just a link on every one of them. So, I;ve turned off minimization, made sure the firewall isn’t blocking any IPs when this error occurs, turned off Brotli, Rocket loader, page rules, etc… down to where the only purpose Cloudflare is serving seems to be providing SSL.