kbin gets occasional updates* so I'd say clear any and all cookies and stored website data for kbin (or, inconveniently, everything), but I assume that at least one of the browsers you've tried hadn't ever logged on to kbin, so that would rule that out.
It might be a browser extension/add-on that's having a weird effect. Try with those disabled temporarily, maybe, if you have any in the first place.
* Not sure precisely what changes, but it might affect cookies. Maybe?