In the extension finder is some code which removes the phpbb_root_path (but only if it is "./") and reappends it for the checks later.

When your front-end file is not inside the phpbb root, but a subfolder and therefor your root path is "../" it does not get removed, but is prepended later again. The the finder tries to locate the extensions in ../../ instead of ../

	/**

	* Removes occurances of /./ and makes sure path ends without trailing slash

	*

	* @param string $directory A directory pattern

	* @return string A cleaned up directory pattern

	*/

	protected function sanitise_directory($directory)

	{

		$directory = preg_replace('#(?:^|/)\./#', '/', $directory);

		$dir_len = strlen($directory);

		if ($dir_len > 1 && $directory[$dir_len - 1] === '/')

		{

			$directory = substr($directory, 0, -1);

		}

		return $directory;

	}

This function in phpBB\includes\extension\finder.php must be modified to replace the first occurance of

'/' . $phpbb_root_path

with

/

and afterwards remove all

/./