We just joined NBN but have been always an Optus ADSL customer. With NBN we often get the "404 File Not Found" notice or "401 Unauthorised" and the latter asks for the router's Serial Number". We have never had this before after so many years.
Very unusual second error message - is it only on one device or all devices? When you connected to NBN, the modem doesn't require your User ID and Password as it did on ADSL - it is set for IPoE/Dynamic IP - does that match your setting? You could also Clear Data Cache and History in your Browser or if possible use a different Browser to see if that reveals better outcome.