Blank page when clicking on unsubscribe

TD;DR: Everything seems perfectly fine, except that people cannot unsubscribe, they are redirected a blank page.

Here is an unsubscribe link for example: link I generated it using the [unsubscribe] shortcode.

Everything seems to work fine except that. Web preview works link, link tracking also. I can even see that the unsubscribe link got clicked, but no one got unsubscribe.

It's an apache server running on ubuntu. I see nothing suspicious in my error log (even with LogLevel debug) and here is how the vhost is configured:

    <Directory /home/sendy_web/www/htdocs>
            Options Indexes FollowSymLinks MultiViews
            AllowOverride All
            Order allow,deny
            allow from all
    </Directory>