When you receive 5xx status or UNKNOWN_ERROR in the response, you should implement a retrying logic. Google has a following recommendation in their web services documentation:
In rare cases something may go wrong serving your request; you may receive a 4XX or 5XX HTTP response code, or the TCP connection may simply fail somewhere between your client and Google's server. Often it is worthwhile re-trying the request as the followup request may succeed when the original failed. However, it is important not to simply loop repeatedly making requests to Google's servers. This looping behavior can overload the network between your client and Google causing problems for many parties.
A better approach is to retry with increasing delays between attempts. Usually the delay is increased by a multiplicative factor with each attempt, an approach known as Exponential Backoff.
https://developers.google.com/maps/documentation/directions/web-service-best-practices#exponential-backoff
However, if retrying logic with Exponential Backoff doesn't help and the error persists for a long time you should file a bug in Google issue tracker
I hope this addresses your doubt!
UPDATE
There was an issue on Google side yesterday (November 6, 2017), you can refer to the following bug that explains the issue:
https://issuetracker.google.com/issues/68938173
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