Error 521 and error 503 cloudflare-nginx, help!

For the last 2 days I keep getting Error 521 (Webserver down) but the server is up and running. If I do a refresh the website is loading fine. Since today I’m getting Error 503 Cloudflare-nginx.

I’m living in the US but the server is in Amsterdam. Website visitors from The Netherlands have no problems.

The server is hosting 30 Wordpress websites. It happens on all websites. See for example https://112amersfoort.nl

Update
The errors are happening more and more. I can’t find any problem on the server. The errors only happen in de US, not in The Netherlands.

Cloudflare servers are whitelisted and not rate limited.

traceroute to 103.21.244.0 (103.21.244.0), 64 hops max, 52 byte packets
1 10.0.0.1 (10.0.0.1) 3.775 ms 5.559 ms 4.040 ms
2 cm-1-acr01.summit.co.denver.comcast.net (96.120.12.13) 13.872 ms 23.295 ms 13.881 ms
3 ae-251-1204-rur02.summit.co.denver.comcast.net (96.110.195.185) 19.036 ms 13.829 ms 14.414 ms
4 ae-2-rur01.summit.co.denver.comcast.net (96.108.139.105) 13.925 ms 15.640 ms 15.087 ms
5 162.151.8.53 (162.151.8.53) 16.011 ms 25.979 ms 39.934 ms
6 ae-501-ar01.denver.co.denver.comcast.net (96.216.22.130) 18.022 ms 24.221 ms 23.620 ms
7 * * *
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19 * * *

Happened for example March 11 11:20 am (MST)

My Uptrends monitor detects 521 errors from other places too like Berlin and Frankfurt.

Anybody idea what to test or do next? Is it possible that the installation of Superpwa on Wordpress is causing these problems?

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!

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