Hey Javier, I'm in the exact same situation. I'm also using Siteground and in my case I accidentally turned on HTTPS using Lets Encrypt. So I've changed http ->https in the config.php and can access the dashboard. However, any admin pages, including the site administration menu doesn't load.
This also means I can't access /admin/tool/replace as this is in the admin folder and for some reason everything in there is not loading up.
Javier, how did you manage to find that rogue http reference inside the .htaccess file?