PayPal Pro IPN returns 403 error
Having reviewed this I believe that your Paypal is possibly configured to point to an older version of the IPN path.

You can change this yourself within your Paypal account, the docs I can see suggest:

To configure Instant Payments Notification (IPN) for your PayPal account Click: Profile, under Selling preferences click "Instant Payment Notification preferences"
It looks like yours is currently set to: https://domain.org/sites/all/modules/civicrm/extern/ipn.php

However, it should now be set as: https://domain.org/civicrm/payment/ipn/3 where 3 is the payment processor id which you can get from Payment Processor screen(Administrator >> System Settings >> Payment Processor). This should fix any new payments setup / made.

For recurring payments that have already been setup what you would need to do is make a change to the site so that those old / failed recurring payments get forced onto the new path, once you've done that you'll need to 'replay' all the previously failed recurring payments from Paypal terminal. Depending on the number of transactions involved, this may involve some work from yourself in the Paypal site again. Or the other method is to use Notification log extension to do replay of failed transaction. 

Cheers

Pradeep