Hi, I think I'm having the same trouble and I was wondering if you had ever found a solution for this. On my site, the "Save" button on the front end saves the edits but doesn't close the lightbox window. To actually see the changes, I have to click the "X" and then refresh the browser. The "Close" button brings up the actual webpage inside the lightbox window, which is really weird.
I have Joomla 2.5.4 and K2 version 2.5.7.
Thanks