I upgraded my website with jupgrade and then reinstalled K2.
Everything looks ok in backend but it is blank page or 404 error in frontend while changing templates.

So, I tried everything and found out that my categories get listed both in current view and in trash view(but without check boxes in trash view).

I have no trashed items.

So I searched in database the trash column but it says "0" to all. I don't see where the problem is.

I found my problem. After update categories' and items' access level was reseted. So I had nothing chosen for access level.