I've just solved a maddening error, so if you get the following error:
Then the likelihood is that your apache settings are preventing the PHP function set_include_path from working (http://www.php.net/manual/en/function.set-include-path.php#92504). I'm using Plesk and had the include_path PHP setting set. This prevents set_include_path from working, which knackers this plugin and quite a few other bits of software.
By unsetting the include_path apache config value this plugin worked again.