Problems Changing Things

My page:
http://unleashed.phpbbweb.com/index....orum=unleashed

It is a guild-forum from the game Archlord.
It was made by Slacked in first place but he left the game and also the forum.
Instead of making the changes he totally left us without doing anything.

So the forum is still on his name, he has the power to remove it, etc.
Ive become leader in the game, and needed to change my rank.
If i go to User Management i cant change my ranking.
It says:
Template->make_filename(): Error - file /home/phpbbweb/templates/error_body.tpl does not exist

And also the previous leader must have normal ranking, for in case he wants to visit the forum, but it gives the same error.

What do i need?

1. I need to transfer this forum to my own data. Slacked ([email protected]) must be no longer in control of it, because he has quited. And i made the forums, not he.

2. I need to know why this error pops up.

Plz help me out...