Reply To: (14) The referrer, relay response or receipt link URL is invalid.

I assume you’re using the authorize gateway.
Here is a tutorial (the 6 steps) about how to allow and define the link to the ipn script. Try to add
in there.
Else they state that if you send a return and an ipn link at the same time it can result in that error. Maybe that’s new and it doesn’t make sense either as both functions are unrelated. We’ll try that afterwards.