Sorry Kyle but I cannot see how you can say purchases seem to be going through fine. From what I can see on the logs when you were testing you were still generating the IPN Communication Errors in the History log and the user subscription was still not being marked as paid and no payments are appearing in the Payments page.
On the Paypal side the IPN History is still showing that they have failed after a number of retries.
Did you get any transaction that completed by processing the IPN and giving a successful notification on the sri-connect site that payment was complete and the user subscription is marked as paid? Or did you always get "Sorry no response for your payment from payment server (error ). Please check your email and status later.
No suitable basket found."
I've retried transactions this morning and the results are exactly the same as before.
The same point of failure still exists and this is when CB Subs tries to communicate back to Paypal after receiving an IPN.
I have looked through the IPN Guide at
cms.paypal.com/us/cgi-bin/?&cmd=_render-content&content_ID=developer/e_howto_admin_IPNIntro#id091F0M006Y4
and can see that after CB Subs receives an IPN it must send back a _notify-validate with (according to Paypal documentations) the exact same fields as received in the IPN in the same order. Paypal will then send back a VERIFIED or INVALID response.
As far as I can see from reading the documentation and the x.com developer site a 500 error at Paypal is just an indication that the correct response to an IPN has not been made.
In the CB Subs code is there any more specific error code identifying what is meant by a "Communication Error" - this description of the error is obviously coming from CB Subs. Does it just mean CB Subs has received a 500 error from paypal?
If there was a list of what would cause a "Communication Error" to be shown in CB Subs that may be helpful?
I did find a very similar issue of 500 errors on IPN at
www.x.com/thread/42429?start=0&tstart=0
. This seems to imply that about March time a change to Paypal resulted in previously acceptable IPN responses being rejected (because of null return values it seems). I don't know if this has been experienced by any other users of CB Subs after that time (I presume you haven't on the joomlapolis site) or whether if you re-ran your testing of CB Subs it would occur but it does appear to provide a possible explanation.
However, to correct that problem (if it is the cause) or from what I can see any other problem would require code changes in CB Subs to modify how it communicates with Paypal. Obviously such code changes may not be a priority for you but I would be interested to know if there was a way to get this done.
The error has occurred on three different sets of Paypal accounts (2 in Sandbox and 1 on live Paypal) and on two joomla sites (the one I have some code mods in and the "clean" Sandbox site you accessed) so although there may be something I am doing wrong and that you missed I am stuck to think of what it is or what other parameter changes could be made.
This site will launch soon and I need to have a working mechanism for taking credit and debit card payments. If necessary my client will pay for a working authorisation to be written for a UK payment processing organisation (or for someone to debug CB Subs Paypal processing to identify problem) as I he will not want to use the US based Authorize.net or the Swiss based YellowPay. Who do I contact at Joomlapolis to discuss this further.