No idea what's causing it. You've a lot of odd caching issues. Seams something installed keeps caching content. You also appear to have AEC installed which is known to corrupt installations (and require a complete reinstall of Joomla).

Not sure what more to advise, there's simply too much installed and too many conflicts for me to debug and especially so when having to fight the constant caching making debugging difficult and inaccurate.

Your ARTIO JoomSEF has caching, but backend won't save due to a fatal JS error so I could not display it and test; maybe want to review upgrading it? Your template is also caching even though caching for it has been disabled.

Disabling K2s user plugin doesn't resolve it nor do I see a K2 authentication plugin. Nor does it do this for me on my local install with latest K2 installed. I recommend reinstalling Joomla and starting over excluding extensions you have no intention of using. For example you've AEC installed, you're using a forms extension to handle subscriptions, etc.. All of which CBSubs can handle safely, securely, and without conflicting with anything.