I could reproduce this bug on multiple browsers and I have no reason to believe this is a client-related issue, nor I think this is a configuration issue of my proxy.

What does your proxy do with the Location: header? As it kinda seems like it's not capturing it and potentially just passing it through

Sams-MBP:~ reedy$ curl -I https://en.wikipedia.org/wiki/Special:Random | grep location
location: https://en.wikipedia.org/wiki/Shea_Hillenbrand
Sams-MBP:~ reedy$ curl -I https://en.vikiansiklopedi.org/wiki/Special:Random | grep location
location: https://en.wikipedia.org/wiki/KTFO-CD

So this isn't our fault if your proxy isn't dealing with it correctly and passing it through verbatim...

In T232213#5471202, @Reedy wrote:

I could reproduce this bug on multiple browsers and I have no reason to believe this is a client-related issue, nor I think this is a configuration issue of my proxy.

What does your proxy do with the Location: header? As it kinda seems like it's not capturing it and potentially just passing it through

I think I don't do anything with it, as you can see from the configuration template that I am using.

Here's the detail (${lang} is the language code, e.g. en ):

proxy / https://${lang}.wikipedia.org {
    header_upstream X-Real-IP {remote}
    header_upstream X-Forwarded-Proto {scheme}
}

Do you think this is a bug of the server I am using (Caddy), or should I change something in the configuration?

https://github.com/caddyserver/caddy/issues/1011 looks a lot like the issue you're experiencing

In T232213#5471251, @Reedy wrote:

https://github.com/caddyserver/caddy/issues/1011 looks a lot like the issue you're experiencing

Thank you for the pointer.

So, in the issue above they suggest to use the preset transparent for the proxy, but I have an issue with that.

transparent is a shortcut that adds the following lines to the config:

header_upstream Host {host}
header_upstream X-Real-IP {remote}
header_upstream X-Forwarded-For {remote}
header_upstream X-Forwarded-Port {server_port}
header_upstream X-Forwarded-Proto {scheme}

I have noticed that I have a problem with the first line of the configuration because if I insert it in my configuration I get an error from Wikimedia's servers:

Domain not configured

This domain points to a Wikimedia Foundation server, but is not configured on this server.

(see screenshot below)

Without the header_upstream Host {host} line the proxy works, but I'm still experiencing the redirect issue described above.

Have you considered asking Caddy for support with their software? :)

In T232213#5472235, @Reedy wrote:

Have you considered asking Caddy for support with their software? :)

Yeah, sorry, I didn't get your initial comment:

In T232213#5471202, @Reedy wrote:

So this isn't our fault if your proxy isn't dealing with it correctly and passing it through verbatim...

I am going to open a bug report to Caddy.

I've fixed this, it seems that Caddy implemented a directive to handle this case, here's the pull request caddy#2144.