Subscription failed to get renewed with "An unexpected error occurred" error

This message usually means that something strange happened in Shopify, which prevented them from creating an order. Usually, it is related to your product configuration, for example the inventory levels, product restrictions, etc.
Most merchants can resolve this issue by opening the subscription in Seal Subscriptions app, clicking the "Edit subscription" button, removing the product from the subscription and then immediately re-adding it back to the subscription and saving the subscription.