When you call result.HitsOrFail it throws an exception that was already captured, meaning that the query failed from the get-go when it was executed. The result.Exception property will have the same exception; it is the one that was thrown.
An HTTP trace should show the POST to the server along with the URI that was used. I would start there and figure out why the server is failing the request. A 503 generally means the service cannot handle any more requests or something similar - you’ll have to debug it. A cancelled task is likely just the result of the service returning 503 and the task being cancelled in the client.
-Jeff