Hello, I'm having this error when I try to send a item from the frontend 403 You are not authorized to view this resource, I have the version v2.5.7 of K2.
I don't remember upgrading, but I think I did >.<
I am working on this site for a long time and I'm also on the law school, so, it's complicated... haha :D

Hi. Try to install K2 2.5.7 to be sure that you have the latest version ( do not uninstall first as if you do you will lose your data). After that ensure that the user who adds the item in the front-end has the appropriate permissions for creating and publishing items.