Were you able to find a solution to this issue. I have migrated k2 content form j 1.5 to j 1.7 and then set a menu item for a k2 item and got the same 403 error

check to make sure the category is set to public. mine were all left blank. I tried doing a mass update in mysql but it did not take the change even though it seems to in phpmyadmin so I just set the categories to public one at a time.

had the same problem but solved it by changing te user permission in K2. First i made a new user group in K2 component and then changed the permission for the individual users. Now it works for me

:)