Got this explanation from Vera.
" Regarding the error " Nest: Failed to login: code=406, status=HTTP/1.1 406 Not Acceptable" this error message it’s caused by the Nest Thermostat and Protect plugin, and it will not go away unless the plugin gets uninstalled, because Nest it’s no longer supported by us. According to the news that was shared by Nest in August 2019, the nest connectivity with third-party apps will not be supported anymore. In this case, you could uninstall the plugin and the sensors will continue to work unless you change your account with google, but we can’t be sure how much the integration will continue to function properly."
Not sure what that means vis a vis the controller responding to alerts from the smoke detectors. I have to test a unit to see if the lights will come on when smoke is detected.