Paypal - Sorry, we were unable to submit an IPN - Django

I am using Paypal with Django. Paypal IPN was working fine two months ago, but now it is failing. And Paypal IPN Simulator gives me this error:

We're sorry, we could not send an IPN.

      

        
        
        
      

    

I am using django with Apache WSGI. Here you have your server config:

<VirtualHost *:80>
    ServerAdmin marcos@domain.com
    ServerName domain.com
    ServerAlias www.domain.com

    WSGIScriptAlias / /var/www/domain/domain.wsgi

    Alias /static/ /var/www/domain/static/
    Alias /media/ /var/www/domain/media/

    <Location "/static/">
            Options -Indexes
        </Location>

    DocumentRoot /var/www
    <Directory />
        Options FollowSymLinks
        AllowOverride None
    </Directory>
    <Directory /var/www/>
        Options Indexes FollowSymLinks MultiViews
        AllowOverride None
        Order allow,deny
        allow from all
    </Directory>

    ScriptAlias /cgi-bin/ /usr/lib/cgi-bin/
    <Directory "/usr/lib/cgi-bin">
        AllowOverride None
        Options +ExecCGI -MultiViews +SymLinksIfOwnerMatch
        Order allow,deny
        Allow from all
    </Directory>

    ErrorLog ${APACHE_LOG_DIR}/error.log

    # Possible values include: debug, info, notice, warn, error, crit,
    # alert, emerg.
    LogLevel warn

    CustomLog ${APACHE_LOG_DIR}/access.log combined

    Alias /doc/ "/usr/share/doc/"
    <Directory "/usr/share/doc/">
        Options Indexes MultiViews FollowSymLinks
        AllowOverride None
        Order deny,allow
        Deny from all
        Allow from 127.0.0.0/255.0.0.0 ::1/128
    </Directory>

</VirtualHost>

      

        
        
        
      

    

In other questions like this , they say the problem is with non-standard ports, but I'm using port "80".

The form I use to submit PayPal is this:

<form action="https://www.paypal.com/cgi-bin/webscr" method="post">
        <input type="hidden" name="cmd" value="_xclick">
        <input type="hidden" name="business" value="paypal@domain.com">
        <input type="hidden" name="undefined_quantity" value="1">
        <input type="hidden" name="currency_code" value="EUR">
        <input type="hidden" name="item_name" value="Title">
        <input type="hidden" name="item_number" value="gD9s3nj4eW">
        <input type="hidden" name="amount" value="26.90">
        <input type="hidden" name="return" value="http://domain.com/thanks/">
        <input type="hidden" name="cancel_return" value="http://domain.com/canceled/">
        <input type="hidden" name="notify_url" value="http://domain.com/paypal_response">
        <button type="submit" name="submit" id="confirmar-compra">Confirmar</button>
</form>