Error with existing app - "There was an error during communication with the service"

We have an existing MT plugin compatible with Studio 2015 and 2017. It was previously working ok, but when validating the user key now (first step in setup) we get the message:

"There was an error during communication with the service. They underlying connection was closed: unexpected error on send."

This is a pretty generic 'we couldn't hit your API' network message but, before we starting digging, can anyone offer a suggestion as to what might have happened to cause this, without any changes on our side?

For 2017, it was an existing install that previously worked. For 2015, it was a fresh install. Both with same behaviour.