For one of the last registrations with payment via PayPal I get the notification 'IPN notification not authenticating at payment gateway' in the backend.
Previous registrations and the last one were working fine without this error.
When I look into the PayPal history I see for the 'problem' registration an error 500 with 15 retries.
When I make the complete URL based on the 'history URL&IPN message' I get INVALID in my Google Chrome browser.
Do the same with the last correct registration I get a blank screen (= OK).
What can I do here to get this issue resolved?
For me all the PayPal settings are OK since this was working fine the last months and nothing has been changed to the website.
