See attached error message. This happened after an upgrade from phpBB 3.2.8 to 3.3.3, initially when reporting statistics but any time going to the ACP General Tab. This function starting on line 2836 in /includes/functions_admin.php failed for the mysqli case:

case 'mysqli':
$mysql_engine = ['MyISAM', 'InnoDB', 'Aria'];
$db_name = $db->get_db_name();
$database_size = 0;

$sql = 'SHOW TABLE STATUS
FROM ' . $db_name;
$result = $db->sql_query($sql, 7200);

while ($row = $db->sql_fetchrow($result))
{
if (isset($row['Engine']) && in_array($row['Engine'], $mysql_engine))

{ $database_size += $row['Data_length'] + $row['Index_length']; }

}

$db->sql_freeresult($result);

$database_size = $database_size ? $database_size : false;

break;

It's probably the database name that threw it off: Bootle-History. Maybe the dash? 

Here is the config.php (password x'ed out):

<?php
// phpBB 3.0.x auto-generated configuration file
// Do not change anything in this file!
$dbms = 'mysqli';
$dbhost = 'localhost';
$dbport = '';
$dbname = 'Bootle-History';
$dbuser = 'Bootle-History';
$dbpasswd = 'xxxxxxxx';
$table_prefix = 'phpbb_';
$acm_type = 'file';
$load_extensions = '';

@define('PHPBB_INSTALLED', true);
// @define('DEBUG', true);
// @define('DEBUG_EXTRA', true);