Server Overloaded

It’s probably not a Ghost related error, but maybe the community can suggest something to me.

Both in the file *_production.log and in the file *_production.error.log I often read "421 Server overloaded"

Often when I connect to my site I encounter Cloudflare 50x errors.

I opened a ticket on Cloudflare, they invited me to read the documentation in here, they said that it is not their problem.

I opened a ticket on the site of my VPS provider and they said that not being a managed server it is not their problem.

Monitoring the status of my VPS it seems that the ram in use is always 1/4 and the CPU is at 20%, there is never a process overload it seems.

Any suggestions?