Hi, i have fixed this by loging into phpmyadmin and emptyed the "online" table.
It showed "Overhead" 14kB.

I need the same thing.

What do i edit in the main.tpl file?

And in the same moment i have disabled this extension:

Colored usergroups ( Extension disabled. )
Created by PunBB Development Team.
Version v1.0.1.

Solved the username error. Now are usernames visible.

Same erron here.

And i have this extension installed:

Admin add user
Created by PunBB Development Team.
Version v1.1.1.