Cloudflare is congested and overloaded, so quick downtime happens almost everyday :/.
I haven't experience this kind of error on my XF site not using CF.
Just a quick tip. If you are using plesk check the /etc/nginx/nginx.conf file for this;
worker_processes 1;
The default value for this is auto and plesk overrides this to limit the number of worker processes.
I am beating myself up for missing this obvious cause, and am making my shame for missing it public.
Why, oh why did plesk think this was a smart idea?
My advice. Set this value to either auto OR if you have high DB/php load to a value half that of the number of cores in your CPU and adjust from there.
My team just informed me about this error. We are all getting the errors. I personally have 6 computers I tried on, different browsers. Incognito, private windows, you name it- it's the same! Not ad blocker.