Did you pull the master or stable branch?
Is there anything in the emoncms log?
Did you try New First User Creation Error - #12 by borpin fix?
Looking into this, the only way I can reproduce the error is if I disable Apache modrewrite, which results in the login screen not being able to find the user/login api end point.
I have now added further error catching to the login screen to display a more useful message:
It either displays a 404 error with a suggestion to check modrewrite or it prints the status code and error message if the error is not a 404.
@dmudo can you check that modrewrite is configured on your system?