Thanks for the heads-up GJC, I need to upgrade my VM but the last time I tried the upgrade failed so I need to set a week or so aside to figure out the problem and get all the testing etc. done ... never enough time.
I should do this anyway, as having updated the IP addresses I no longer get the IPN failure, in fact I get no notifications at all from PayPal, and no errors in the web server logs, the Joomla logs or the VM logs, so this is a significant problem right now. Rolling back the IPN code checks makes no difference either, as I guess the "error contacting IPN servers" is somewhat random based on the IPs that are actually alive at Paypal. I've successfully updated my test site to J3.9.26/VM 3.8.9, and I see none of the new IP Addresses in the paypal.php, which is a bit worrying.
However the "IPN Provider IP" works on the test site so I may do something about this on my live site too (in the database) if it temporarily fixes my problem.
What's the actual effect of disabling the "IPN Provider IP" ? Do I lose any callback data like Paypal fees/status, or is it "only" a security feature to stop payments being spoofed as confirmed ?
David
EDIT: After a long night, I've finished testing the test system, upgraded the live site to J3.9.26/VM 3.8.9 too and everything is working very well with "Check IPN provider IP" enabled. I haven't examined the code where the IPN IP addresses are actually checked so I don't know if that's been fixed or if I'm just lucky so far.