Can you confirm again what you have for the commit hash underneath the version number on the /admin/plugins page?
And is this for a subscription or a one-off purchase with the pricing table?
I did add a new setting for the subscription plugin for debugging. It is called discourse_subscriptions_enable_verbose_logging. Once that is enabled you can visit /logs and try the checkout process again and see if you see any output from it. Feel free to send me some screenshots of it in a PM and can help debug things further for you.