PHP-FPM and NGINX with Wordpress redirect (sometimes) to weird url and blank page

I followed many tutorials here and thanks to the community I was able to set up a LEMP stack, running on Debian 10 with Nginx 1.18, php 7.4.9, WP5.5.1 Multisite. Everything is working very well but one thing, which is driving me crazy. I have no errors and all the redirections work flawlessy but whenever i got into the admin page and i try to create a new site… it redirect to the very same page (like a refresh) and nothing happen. The url in the address bar is weird:

/wp-admin/network/site-new.php?action=add-site%3Faction%3Dadd-site

The same problem appears whenever i try to upload a plugin or a theme… blank page and that url:

/wp-admin/network/update.php?action=upload-theme?action=upload-theme

And again… let’s try with a plugin! Err… NO, again a blank page and the strange duplicate url:

/wp-admin/network/update.php?action=upload-theme?action=upload-theme

Here’s a closer look to the console POST INFO:

POST https://op.org/wp-admin/network/update.php?action=upload-plugin
scheme	https
host	op.org
filename	/wp-admin/network/update.php
action	upload-plugin

Yes! that’s cool. Let’s see what’s next.

REQUEST HEADER
Host	op.org
Origin	https://www.op.org

Seems legit, you know where you are. How do you think he’s going to answer to this?

HEADER ANSWER
location   https://www.op.org/wp-admin/network/update.php?action=upload-plugin?action=upload-plugin

Wait… there’s something wrong here… update.php?action=upload-plugin?action=upload-plugin Oh… there’s some sort of echo? a deaf client assistant-rewrite?

I’m hitting a wall here, I don’t know where to look for errors, there’s nothing in error_log. Php ini’s, fpm config and nginx blocks they all look just fine to me. What could it be? Please help me, I feel like my head’s about to explode.