Sorry, it’s been a while since I took a look at this. I don’t think the thread you linked applies to me.
Firstly, I am not getting a 404 error, I’m getting a 422 error.
Second, the file you mentioned (login.jst.eco) is up to date with the line you’ve highlighted
Third, the “Google” button is rendered properly as a form with action /auth/google_oauth2
I am using the docker-compose installation directly from https://github.com/zammad/zammad-docker-compose with no modification.