W3 validator URI checker giving HTTP resource not retrievable error


When using the Validate by URI checker on https://validator.w3.org/ on a website on a fresh installation of Almalinux 8 + ApisCP I’m getting the following error

IO Error: HTTP resource not retrievable. The HTTP status from the remote server was: 400.https://site.tld/

If I move the website over to another setup, such as simple nginx or apache stack or put the site behind cloudflare the error goes away. I’m not finding any obvious cause for this and why it only happens if the site is on apiscp but goes away when proxied through cloudflare.

This is caused by sending a HTTP/1.0 request, which is a common pattern of abuse by spam bots. Most human traffic is sent using HTTP/1.1 + keepalive or HTTP/2.0 in the case of a TLS-encrypted channel.

This can be disabled globally or per-site.