Hell if I know. I was able to get the same popup window with IE, but only using the https address. (Even then, I could make it go away & type normally without a freeze. I assume that's kind of a separate issue.) Anyway, I guess the main question is why your system insists on treating it like a secured site. (If you had some kind of setting to redirect you even when you type it in right, you'd still see that "s" in the location bar, I assume.) I like puzzles (or I wouldn't be playing wordtwist), but this has me stumped. I hope someone else has a clue.
You could always try the other recommended fixes for certificate errors, like making sure your system clock is OK or resetting IE, but I get the feeling those aren't really applicable. (Updating IE may be an option, especially since that's what controls certificates in Windows, I think. Maybe it will do something?)