What payment method did they use? Visa? I'm not seeing any such issues. That error implies their basket doesn't exist anymore. If they take too long during the purchase process it's possible the basket expired. Ensure the basket lifetime is set to something reasonable in CBSubs > Settings > Display > Baskets. For frontend the default expire time is 3 hours. So I guess if they sat there for 3 hours, then paid, it may try paying a basket that doesn't exist but that should've been stopped at CBSubs it self.

Please be sure "Payment Button method" is set to "Inline Form (Elements)". If you're using any other mode then let me know. Do not use server side unless you absolutely 2000% have to.

I've retested subscribing to recurring and non-recurring during registration and after logged in as working fine. I don't even see how that's even possible. They should never leave your site so it's not possible for typically hosted page redirect issues. If you've any form of site caching please purge it as maybe the Stripe JS is being cached or something of the sort. None of the changes in latest release would even be able to cause such an issue.