Shopware AboCommerce for Recurring Payments
With Shopware, the way this works is by triggering a server-to-server request in the background when a recurring transaction is made. This can cause a 503 or similar error to be displayed if an error occurs while executing the recurring transaction. The reason for this can be a number of things so the search for the root of the problem will seem difficult and tedious.
If there is a transaction / order created, then you would be able to find more details in the logs. This is similar to debugging, as with other errors. However, if no transaction exists or it was created cleanly, then the error is probably due to a general problem. This should be written in the server log or the Shopware log.