"Bad Response Form Submission"

I've just begun a Drupal 6 site and I've installed the Popups module. Right now there's no other modules interacting with it. I've created a custom content type that can be created by certain user roles, and I'm trying to have that creation form appear in a popup. There's no modules installed that handle forms; it's the Drupal default.

Whenever I test this out as soon as I submit the form it says "Bad Response Form Submission." However, the content is still created perfectly; I just can't get past this error. The link loading the page is set to the "popups-form-reload" class. To make things more interesting, I tried this patch here and it didn't work; I deleted the code for this error out of the .js file and it still appears. Any ideas?

On another note, I'm brand new to Drupal and this website / community... I did my research, but forgive me if this is an easy question!