Bug on validation

Hi guys, I have a bug on the validation process.
I created a page with the [register ..] shortcode inside. When I try to register with an email that is already used by a registered user, the plugin understands that there is an error (I mean: in simplr_form_function.php:478 the sreg->errors contains the record with the relative error message), then the process redirects me correctly to the same page but instead of the form with the error messages I see the content of a 404 page. I mean: the url is the correct permalink of the page with the [register ..] shortcode, but the content is a 404 page.
If I enter a new email address the process goes on correctly and I’m redirected to “Thanks” page. This bug happens just when I use an already known email address.
Does anybody what the hell is going on?
Thanks a lot!

http://wordpress.org/plugins/simplr-registration-form/