Form submission causing a 404 error

Hey all,

I have a form on a page that, when submitted, should take the user to a page called ‘Thank you’ (https://mysite.com/thank-you/).

If I manually type this and go there the page works fine, although it shows an error telling me that I should not be there (because $_POST['action'] is not set), but when I submit the form I get a 404 error, even though the address in the address bar is exactly as it should be (https://mysite.com/thank-you/).

Has anyone come across this problem and a solution before?

Thanks.