Here’s what I think is happening:
I assume we are all on multisite installations.
On a multisite installation, wp-login.php?action=register is redirected to wp-signup.php. This plugin uses wp-login.php?action=register no matter what.
I tried changing the action part of the form and made other modifications to make this work with wp-signup.php, but I’m not skilled enough to make it work.
Maybe someone else has an idea?