I cannot increase the maximum number of private messages for the inbox. A bit of digging led me to this function in \includes\functions_privmsgs.php

function phpbb_get_max_setting_from_group(\phpbb\db\driver\driver_interface $db, $user_id, $setting)
{
if ($setting !== 'max_recipients' && $setting !== 'message_limit')

{ throw new InvalidArgumentException('Setting "' . $setting . '" is not supported'); }

// Get maximum number of allowed recipients
$sql = 'SELECT MIN(g.group_' . $setting . ') as min_setting, MAX(g.group_' . $setting . ') as max_setting
FROM ' . GROUPS_TABLE . ' g, ' . USER_GROUP_TABLE . ' ug
WHERE ug.user_id = ' . (int) $user_id . '
AND ug.user_pending = 0
AND ug.group_id = g.group_id';
$result = $db->sql_query($sql);
$row = $db->sql_fetchrow($result);
$db->sql_freeresult($result);
$max_setting = (int) $row['max_setting'];
$min_setting = (int) $row['min_setting'];

return ($min_setting > 0) ? $max_setting : 0;
}

When I execute the SQL query manually the result for $min_setting is 0, and $max_setting is 2000

I suggest to change the last line in the function to:

return ($min_setting > 0) ? $min_setting : $max_setting;