check in index template - there may be repeated code related to prostats plugin. if so, remove the duplicate
one {$ps_header_index} should be before {$forums} and one {$ps_footer_index} after {$forums}
check for correct database details in
~/inc/
config.php file
$config['database']['type'] = 'mysqli';
$config['database']['database'] = 'xxxxxxx';
$config['database']['table_prefix'] = 'mybb_';
$config['database']['hostname'] = 'xxxxxxx';
$config['database']['username'] = 'xxxxxxx';
$config['database']['password'] = 'xxxxxxx';