I had tried with same with baserow docker (and also with automatisch docker)… it works.
I am also not able to sign up a new user without invitation (but I think this is the a limitation of the self-host plan).
Yes when I was clicking on the invitation link it accepted and redirect me to the login screen (I posted in my first post) . I recored it with jam as video - will send it to you by PM. So much thanks for taking a look on this.