So it's strictly only the recurring plan that's failing? What price and duration are you using on the plan? Are you using a different initial price and duration from the regular price and duration? If so what are those values as well? Owner ID wouldn't make any of this fail so there's no reason to suspect that parameter is causing this. It seams like some sort of configuration problem, but unsure where.

We now have two test user records - they are very similar but distinct. One gets through to paypal (regardless of browser, btw) and the other doesn't! We've compared Joomla and CB info and we can see no obvious differences that might affect payments... Any ideas?

Is both their usernames just plain alphanumeric characters? If one has special characters it maybe the basket item line being sent to PayPal with their username in it causing it to be rejected by PayPal.

Are there any logs for the failed payment attempt in CBSubs > History Log or failed notifications in CBSubs > Notifications? If so please PM backend super administrator login credentials and will review those logs.

Strange, I don't know what more to suggest. Everything from CBSubs end looks fine. Please PM backend super administrator credentials as well as your test user credentials where this is failing and will take a look, but it's most likely something failing on PayPals end; probably just a setting in your seller preferences.

Have reviewed your install and appears to be failing due to the zip code. PayPal doesn't seam to like the postal code format of "## ###", which is valid for the postal code of the user. This information isn't really relevant or necessary for PayPal though since recurring payments require a PayPal account. I've turned off sending of that information to PayPal and seams to have resolved the issue. I've also created a bug ticket as that ideally should be working fine and could be an API usage issue on our end.

forge.joomlapolis.com/issues/8023