Just think I knew what is wrong, do you use the name of the App as in the example? This does not work as you don't have qvf files on Qlik Sense Server. The name has to be the app ID (see Obtaining an app id via the Qlik Sense hub)
with regard to this particular 'OpenDoc' call, I've only ever set the appid. For the same Engine API code, there have been issues, and then, there haven't. And then an issue. And then not. No code changes.
As for the app, I'm afraid I don't know specifically - I'm merely a consumer of it. However, I'm trying to find out from people who manage it as to what is going on.
The error information I pasted in at the top is definitely what I'm getting back
The app is currently version 1.0.2, but the server will be upgraded to 1.1 in the coming days.
here is a screenshot of the websocket request/response... note that it is working at the moment, but who knows for how long. I conclude that it's not a code issue, but if somebody could decipher the error message it'd be appreciated.