The error message isn't really helpful. It could be some plugin, entirely un-related to K2.

In any case, which K2 version are you using and on which PHP version?

You can try upgrading to K2 v2.10.4 (dev) from getk2.org/downloads/?f=K2_Development_Release.zip (it's safe to do so). If this does not solve your issue, then it's some plugin to blame, probably something which is surfaced when K2 is loaded as well. Alternatively, it could be some old K2 template override.

In any case, have a look at your site's server log for the actual error or switch "error reporting" to "maximum" in Joomla's "Global Configuration" and just reload that page.