Have been extremely busy the past few months, so accept my apologies. But since this isn't going to change anytime soon, all I can say is that you'll have to do some digging on your own. E.g. examine the JSON (XHR) response using your browser's console. It's most likely some 3rd party system plugin injecting bad code onto the JSON output of K2. It can be a PHP notice for example. In most cases it's easy to double check by setting error reporting to none in Joomla's Global Configuration.
Fotis / JoomlaWorks Support Team
Please search the forum before posting a new topic :)