Registration doesn´t work at all

I have installed this plugin, but somehow it doesn´t work.

If I fill out all fields and press the submit button, it loads the same page again in the domain-field of the browser but shows the content of the 404-error-page.
Sometimes it shows just a white screen with the words “security check” in it.

I really can´t figure out why this is happening.
The users are not listed after submitting the form.

Do you have any idea what this could be?

http://ww.wp.xz.cn/plugins/simplr-registration-form/