I'm having the same problem.
Using a component developed by me, as soon as i initialize my controller, the k2 content module throws that error. even if i don't initialize the controller..
I've found a workaround adding :
JModel::addIncludePath(JPATH_SITE.'/components/com_k2/models/','K2Model');
to the mod_k2_content/helper.php file on:
class modK2ContentHelper
{
public static function getItems(&$params, $format = 'html')
{
jimport('joomla.filesystem.file');
..............................
class modK2ContentHelper
{
public static function getItems(&$params, $format = 'html')
{
JModel::addIncludePath(JPATH_SITE.'/components/com_k2/models/','K2Model');
jimport('joomla.filesystem.file');