CSRF error on login

Well as I said before, I tried the network tab, and the only new info I got was the response code (403 Forbidden), and that doesn't really help much (I never explicitly set that header in my code, so again the code that causes this seems to come from CI itself). I did not check the JavaScript console, because there is no JavaScript on the page. I did check to make sure that the hidden input field with the CSRF token was there (it was) but it's just a simple HTML form (no AJAX calls or anything that would print to the JS console).