I could replicate the Error 522 on one site! www.two2go.ch
I cleared all caches on the website (fastcgi & redis) and then also purged everything on Cloudflare.
Then I typed the domain into an incognito window and it took forever to load. So I reloaded the site and it took really long to finally load. I scrolled down a bit and clicked on a post. Then I got the Error 522.
After that, every page loaded fine and fast.
I screen recorded it and uploaded it to YT: T2G E222 - YouTube