IPN returns with no data

[jonrmorgan]

My IPN back-end code has worked fine for several years, but recently I started receiving IPN messages with no data.

Here's what happens:

1. Transaction completes OK. Customer clicks on return to merchant link.
2. Paypal sends an IPN message to my success page (verified in server log) with no data. Nothing to repond to.
3. After a few secs. PayPal switches to another page with Return to Merchant link. Customer clicks and is sent to my Cancel page.

Here's what I've tried so far:

1. Verified on my account's IPN History page (Live and Sandbox) that messages are sent with IPN data.
2. Checked my back-end code on LocalHost - all fine.
3. Resent messages from the IPN History with the same result - no data arrives.
4. Confirmed with WSP that server firewall is not blocking/stripping data - if it were blocking PayPal the messages would not be logged.
5. Confirmed server is TLS1.2 compliant.
6. Ported my site to another WSP to test with same result - no data arrives.

I'm pretty much out of ideas on this one !

Does anyone know how to confirm that IPN messsage data is received at the server - or have any other ideas how to fix this problem ?