Did you solved this already?
I’ve got the same issue after upgrading to 0.84.2.
First the 403 error. I unlinked the Home Assistant test app. Linked it again. I was able to sync devices, But now I start Home Assistant I get “Could not contact Google for request_sync”
I had a lot of trouble and ended up deleting the project in the Google Console and creating it again, then linking in the Google Home app. I haven’t tried another request_sync since the unlinking / relinking updated the entities anyway