Django Paypal IPN : bad content type only in live mode (it works in sandbox)

Question

I have successfully integrated django-paypal in sandbox mode, along with Django debug settings. My payments are done, and the signals attached to valid_ipn_received are triggered.

When I move to the PayPal live mode, payments are also received, but I don't receive any signal anymore. Here is the code that normally triggers PayPal IPN:

class StoreItemView(SomeUserCheckMixin, SomeAjaxMixin, View):
    # …some business logic…

    paypal_dict = {
        'business': settings.PAYPAL_RECEIVER_EMAIL,
        'item_name': 'my item',
        'amount': '20',
        'discount_amount': '10',
        'currency_code': 'USD',
        'invoice': generate_uuid_as_string(),
        'payment_date': timezone.now().strftime('%H:%M:%S %b %d, %Y') + ' CET',
        'lc': 'FR',
        'bn': 'SomeCompany_BuyNow_WPS_FR',
        'email': request.user.email,
        'first_name': request.user.first_name,
        'last_name': request.user.last_name,
        'address1': request.user.profile.adress.street,
        'address2': request.user.profile.adress.extension,
        'zip': request.user.profile.adress.zip,
        'city': request.user.profile.adress.city,
        'country': 'FR',
        'night_phone_a': '33',
        'night_phone_b': request.user.profile.phone_number,

        'notify_url': notify_url,
        'return': return_url,
        'cancel_return': cancel_return_url,
        'custom': request.user.username + _separateur + str(cible),
    }

    return render(request, self.template_name, {
        'form': PayPalPaymentsForm(initial = paypal_dict)
    })

# After my functions to hook to the Following signals
valid_ipn_received.connect(store_check_transaction)
invalid_ipn_received.connect(store_alert_bad_transaction)

My config settings are set so that ADMINS contains mails to alert if any error occurs, some logging messages shall appear in my custom logger (and I know it works for other operations on my website) and my PayPal settings are set like that:

# In debug mode:
ALLOWED_HOSTS = ['localhost', '.paypal.com', 'paypal.com.']
PAYPAL_TEST = True
PAYPAL_RECEIVER_EMAIL = '[email protected]'
PAYPAL_BUY_BUTTON_IMAGE = 'https://www.somecompany.fr/static/media/pay.png'

# …and in production / live mode:
ALLOWED_HOSTS = ['.somecompany.fr', '123.234.34.56', 'paypal.com.', 'paypal.com.']
PAYPAL_TEST = False
PAYPAL_RECEIVER_EMAIL = [email protected]'

Nothing about a signal appears. What did I miss? Is there a way to check where my IPN calls end? I found nothing on the PayPal developer dashboard.

Update: I receive this assertion error, but only in live mode.

AssertionError: Invalid Content-Type - PayPal is only expected to use application/x-www-form-urlencoded. If using django's test Client, set `content_type` explicitly

I know it receives json instead. Is it normal?