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

<input name="name" type="text" />

and then you have to chose between getting rid of it or renaming it.