This happens due to internal error of Sentinel Hub. Ideally your application/script is done in a way to repeat the request (e.g. twice, not indefinitely) if you get 503.

More specifically - there are several (tens of) servers running to process all requests coming to Sentinel Hub (last month about 130 MIO requests) and occasionally enough of these servers die (they all commonly die for one reason or another, and are then recreated) that the request cannot be processed. Then there is either 500 or 503 error.