Actually, Easyblog doesn't handle the user registration, the Easyblog welcome module only shows a registration link to the Joomla registration page https://blog.madridactual.es/component/users/registration?Itemid=1091 .

In order words, your site Joomla registration doesn't work now.

Can you try to turn off your Nginx server cache and see how it goes?

Because I suspect probably it's happening because the CSRF token is cached on the server so it always loads the same token on the webpage.