multisite /network/sites.php redirects to /

To me it seems that you have an nginx server redirect in place that might be causing the issue.

When I curl /wp-admin/network/sites.php on other hosting providers, I’m correctly 302 redirected to the login page, and that redirect is done by WordPress. I can see this in the response headers. For example:

$ curl -Is https://pressable.com/wp-admin/network/sites.php | grep "edirect-"
x-redirect-by: WordPress

That’s just one of several requests I made to WordPress sites, and all of them had the x-redirect-by parameter in the response headers.

When I curl the same URI on your server, this is what I see:

$ curl -I https://wtnschp.be/wp-admin/network/sites.php
HTTP/2 302
server: nginx
date: Sun, 23 Feb 2020 16:44:08 GMT
content-type: text/html; charset=UTF-8
location: https://www.wtnschp.be/
vary: User-Agent
host-header: b7440e60b07ee7b8044761568fab26e8

There’s no x-redirect-by: WordPress header. What this indicates to me is that there is some rule in your nginx configuration that has this redirect in place. I can’t be 100% sure, but generally speaking a redirect coming from WordPress (using wp_redirect()) would show up in the headers.

So, I’d recommend looking at your nginx conf file to see if the redirect is there and/or contacting your host for further support on this issue. I hope this helps! 🙂