Fix a PayPal script which is returning a 404 Error for the IPN

I am using a Wordpress plugin called 'Donate Plus'. This plugin hasn't been updated in a while, and some users have been having difficulty with it, including myself. I have fixed all of the issues, apart from one, and that is that I am getting a 404 error for the IPNs, i.e. when I go to look at my IPN History in PayPal, all transactions show as 'retrying', with HTTP response code 404. As I understand it, a 404 error means that the page can't be found. The Notification URL is correct: [login to view URL] And the page exists! It has also been suggested to me that the 404 error can be caused by the script not returning the same data to PayPal that it receives. I was told to check the server access logs as this might help with checking this. I have looked at the raw access logs in cPanel (not sure if this is the same as the server access logs) and there is no useful information there. While payments are getting sent through OK, multiple thank you emails are getting sent out and multiple instances of the same payment are getting added to the Donation wall on my website, which I presume is related to the IPN retrying.