I think I'm at the last step to get IPN working, but this one seems impossible to solve. My Payment Notification gets sent to PayPal correctly, but can't seem to reach my server. On the IPN History I always just see "Status: Retrying" and "HTTP-CODE: 404". Well, I know what a 404 error is, but the Server and the address IS REACHABLE 100%. I tested it with https and without. I even copied the address the IPN History told me it tried to reach and I found myself on the correct page.  So it can't be a 404 Error right? What else can I try? 

I tried it with the IPN SImulator too, but that always results in "IPN was not sent, and the handshake was not verified. Review your information.", which seems to be a bug from PayPal itself, because it doesn't even matter what URL I type in.