503 Service Unavailable

The web server responds with this status code when it is currently unable to handle the request due to a temporary overloading or maintenance of the server. This is a temporary condition, which will be alleviated after some delay. If known, the length of the delay may be indicated in a Retry-After header. If no Retry-After is given, the client should handle the response as it would for a 500 response.

Possible solutions

You should check milliseconds provided in the Retry-After header and retry the request after that period. In case the header does not exist you should retry in a few seconds.
In case if you are still getting the same status, please, contact API support team for more assistance.