Web hook no longer works for present events while in production.

Actually  subscription was not renewed and expired.  I restarted app, created new subscription. 

When renewing subscription my app gets the list of extensions; to do that the app renews access token and gets this error

refresh token:{  "error" : "invalid_grant",
  "errors" : [ {
    "errorCode" : "OAU-202",
    "message" : "Refresh token was not issued for this application"
  } ],
  "error_description" : "Refresh token was not issued for this application"
}


What is application in this context?

Yes. You are correct. We updated our app and got new app key. So we need to shutdown app and restart using new app key. A bit of a hassle when you upgrade your app.
Thank you for your help.