Google SmartLock for web seems to have a broken disableAutoSignIn() function. This used to work but about a week ago the url the function calls to sign you out is now returning a 403 error.
https://smartlock.google.com/iframe/request?client=xxx&id=xxx&renderMode=navPopout
I am also getting this error on other sites using SmartLock such as Hipmunk
https://developers.google.com/identity/one-tap/web/retrieve-credentials
Sorry, this a bug on the Google side, a fix just rolled out, let us know if you still have trouble.