Granted the issue you are experiencing is related to the Subscriptions API, it is necessary that you share more details about the error you are experiencing for the community to be able to help.
A redacted error that hides personal or business information would be a good starting point. But given this is happening for a specific customer, feel free to DM me with details on your shop, the subscriptions application being used, the order/customer and the error (e.g. screenshots, copy/paste, etc.).