Hello, turn the file permissions for the libraries folder back to 755, this is a big security risk for your site.
I think that the problem may be caused by the SEF extension you are using. The fact is that when I click the link you gave me it gets redirected to this URL.
http://www.afdekproducten.nl/component/option,com_googlebasexml/catid,27/format,xml/lang,nl/product_country,NL/product_currency,EUR/
It looks like this is confusing Joomla and it is then trying to render the page as an html document when is should be XML, so it is looking for an HTML template that does not exist.
You should turn off redirection for GooglebaseXML URLS. For example if you are using SH404SEF then it has an option to turn off URL redirection by component, you should do this for the googlebaseXML component.