My Magento 2.1 installation seems to have issues with redirects after a transaction has been completed. For example, registering as a customer gives a 503, placing a order gives a 503 and so on.
When viewing the back-end the transactions have been finished, when you refresh after register 503 you are nicely logged in and such. When your checkout is still spinning the order has already been added in the back-end. Any clue?