bad news: the payment goes through all the way to jTicketing which in turn invokes my plugin, and seeing the green bar means everything is fine i.e. Stripe is able to invoke the webhooks, and they inform my plugin that the payment was ok, and the result is stored correctly in the temporary folder. Then my plugin sends back to jTicketing the confirmation and we get possibly the hardest error to catch.
I checked their code and line 84 of components/com_jticketing/controllers/payment.php seems the origin, the error is thrown simply when the 'processing' param is found empty.
It's my plugin not passing the 'processing' param as you mentioned. The return url is provided by jTicketing
I went to look at my test server and I could not see the 'processor' field because SEF is enabled, and maybe the processor is set as default by the router.php which is not invoked without SEF.
This is just a guess, but I added the processor and pg_plugin attributes to the submiturl. pg_plugin is another possible responsible for not having the processor carried through.
I will try to attach the version here, as it doesn't make sense to release it yet if it doesn't fix the issue.