form action=<permalink> goes to correct URL but generates 404 error?

i’ve created a page through the wordpress interface which contains a contact form. when the user submits the form he/she is taken to “www.mydomain.com/thank-you/” which is the permalink address of the ‘thank you for submitting the form’ page. the form submits and the browser goes to the desired URL and i get the proper template layout, but instead of the page text i get “error 404 – not found”. if i then click in the browser address bar and hit return to reload the page i get the proper “thank you for filling in the form” text.

anyone any idea how to get around this?