从搜索引擎访问时不会加载Joomla站点

My site loads fine when accessed directly http://www.holidayexpress.org/

However, when accessing the site via a search engine such as Google or Bing, it fails to load and gives only this message:

Illegal variable _files or _env or _get or _post or _cookie or _server or _session or globals passed to script.

I have researched the message but wasn't able to find any solutions.

Can anyone tell me how I can fix this or what's causing it?

I have also compared the HTTP requests and I notice the only difference seems to be the Referrer header:

Host:www.holidayexpress.org
Referer:http://www.bing.com/search?q=holiday+express&go=&qs=n&form=QBLH&pq=holiday+express&sc=8-15&sp=-1&sk=&ghc=1&cvid=e26b4f1500b4420eae67e6914a8bfa33
User-Agent:Mozilla/5.0 (Windows NT 6.1; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/29.0.1547.76 Safari/537.36

Again, when accessed directly, it works fine. But, if you try to access it via a search engine it won't work!

Thanks for your help.