Impossible to subscribe

Hi all,

When users try to subscribe they get this error:

Oops! There is a problem with this form:
textStatus:error
errorThrown:Not Found
responseText:

The subscribe button is on the home page here:
http://blog.zerohungerchallenge.org/

Steps taken:

1. I’ve updated plugins, template, etc
2. It is not linked to the mod_security (added htaccess etc…)
3. It was already not working prior to 4.5WP update
4. iframe or html versions result in the same error (creating a new form too)

Note: for those that had subscribed in the past, it still works fine and sends out emails correctly.

https://wordpress.org/plugins/wysija-newsletters/