Enabled Google Auth for existing users

You may be using an unsupported flow through Lock or just have it misconfigured; the automatic enroll/setup implies that user is doing authentication as part of a redirect-based flow where the user-agent can be redirected to the enrollment page. I would advise you to create a new question detailing what exactly you’re using (Lock version, options, types of connections, etc) and also any error information you may have.