Thank you for responding and your tips!
We host our 30 websites on a powerful Virtual Private Server (PerformanceVPS) in Amsterdam. We checked all our settings and logs. The Cloudflare servers are whitelisted and not rate limited. Our server has not reached any limits.
I work from the US (Colorado) and have these errors way more often than my visitors in The Netherlands. I user different browsers and test from different locations.
My websites are not loaded correctly, images are sometimes missing. I see these errors:
Failed to load resource: the server responded with a status of 503 ()
Failed to load resource: the server responded with a status of 521 ()
When I change Cloudflare to development mode to bypass cache these errors about loading resource are gone. I still sometimes get the Error 521 Browser → Cloudflare (Chicago) → Host (error).
Any suggestion is appreciated!