404 pops after custom form submission by POST

Short version
I submit a custom form through POST, and WP returns unexpected 404.
HELP!!

Detailed version
I have a custom form at a my WP blog. An example here: http://berg.com.ua/seminars/?order=1

I am making a related seminar database, and users will be able to apply through such forms.

I creater my custom template, created a page with that template, and have all the code in that template, and all the client-side functionality on that seminars page. Browsing done via GET parameters.

The code for the form is meant to send email, and return the user to the current page. However, WP returns 404 error. The problem is that the URL does not change before and after form submission (except for the #client-form at the end).

Please help avoid that problem. I need not that mistic 404 on my custom page after POST form.