400 Bad Request

Not sure. I've never seen or heard of that issue before. I just tested on your site and couldn't recreate either.

Doing a quick Google Search, it sounds like it's a corrupted cookie issue with Google Chrome. The general suggestion is to delete cookies/cache, like you mentioned. The only other suggestion I'd have to test is to try going into icognito mode (Cntrl-Shift-N) and test again a few times there. I'm pretty sure cookies from that mode are stored in a different directory, so that may help narrow down if you need to physically find the cookie location and delete the files manually. If they are corrupted, it sounds like the deletion process inside Chrome doesn't always work.

Test and let me know how that goes.

Thanks,
Alex