To reproduce this bug as admin you need to perform the following steps:
- The user subscribes to a PTA
- The user cancels the subscription from his settings
At this point the user unsubscribes correctly and can re-subscribe, but if the next step is performed, the error occurs.
- The administrator removes the unsubscribed user from the Stripe Customer panel.
When doing this last step which apparently can also be done by users in some way (I don't know), so I have described it in a way that can be replicated by the administrator, if the user tries to re-subscribe he will get an API error with any payment attempt after accepting the payment terms.