It's because your server is running scripts from aliased directories. Dreamhost and other similarly configured hosts have this issue.
Comment out this code from gallery2.php:
Code: › elseif (!empty($pluginStatusList['rewrite']['active'])) {
$template->assign_block_vars('switch_phpbb_base', array(
'PHPBB_BASE' => strtolower(substr($_SERVER['SERVER_PROTOCOL'], 0, strpos($_SERVER['SERVER_PROTOCOL'], '/'))) . '://' . $_SERVER['HTTP_HOST'] . substr($_SERVER['SCRIPT_NAME'], 0, strrpos($_SERVER['SCRIPT_NAME'], '/') + 1))
);
}
You will probably also have trouble with the image selector, because it won't be able to build a correct path on it's own either. You'll have to hard code a path into the mod for bbcode.php if it doesn't work properly.
The config error stems from the same path discovery problem. Go to the config screen and have a look at the paths that got filled in automatically and make sure they are right. Especially the path to embed.php