betterAuth.api.getAccessToken on the server side. The error seems to happen when the access token expires and has to be refreshed. As far as I can tell stateless mode is unable to refresh access tokens correctly, maybe because account_data cookie doesn't have the needed refresh token or some other reason. For us this is unfortunate, since we had automatic token refreshing working under next-auth. Happy to help debug further, need to get a repro going, but figured I'd at least give the feedback first.