Steps to reproduce:

1. Login as Admin and open Zabbix frontend in two parallel tabs
2. In the first tab open Administration - Authentication
3. In the second tab - log out
4. Return to first tab and click Update in Authentication form
5. See "You are not logged in" error message:

   You are not logged in
   You must login to view this page.
   If you think this message is wrong, please consult your administrators about getting the necessary permissions.
   

6. CLick Login and enter Admin credentials
7. Observe error message:

   Class not found for action .
   

Expected: user is successfully logged in and redirected to Authentication form, or at least to Dasboard.