The solution is to add this line

<? db_connect( $g_hostname, $g_db_username, $g_db_password, $g_database_name ); ?>

after the <? login_cookie_check() ?> line.

I'll try to get a quick fix out soon. I was removing unnecesary code and apparently got too eager and didn't test every single thing. Yuck.

Same error occurs when trying to delete a user from the manage section.

Same fix but for file manage_user_delete_page.php3

Fixed in 0.13.1 or just manualy add the two lines.