I recently needed to make a custom register form for a Wordpress website, so I used a custom template with both form and treatment to make it simple. But, I came across a rather annoying bug : the form would constantly result in 404 error.
The solution is quite simple : you can't use a "name" field in your form. If you do, the field is interpreted by Wordpress and you end up with an error. So you will have to look for
and then you have to chose between getting rid of it or renaming it.