Security Heartbeat is not yet fully enabled in Sophos Cloud, but we are working on getting the final bits in place.
We will post an update ones we have everything in place.
I'm on beta2, in mid october, and though initially heartbeat worked with my sophos cloud account, it has since stopped and now I cannot get my coppernicus to reregister as I receive either timeout errors, or same as above complaining of temporary server error.