I am having the same issues, on Joomla 2.5.7 and just upgraded to K2 2.6.0 and now all of my K2 items give me 403 errors.

I started a new thread, I did not want to take over this thread
getk2.org/community/New-to-K2-Ask-here-first/163286-403-errors-on-all-items-after-260-Upgrade

Feel free to merge them if needed.