Hi evn64,
Thanks for purchasing and using my plugin. From the description you give above, I think the solution may lie in the .htaccess file. However, I am not an expert in editing that. Nonetheless, you may have to give some more information. So far, I understand that you have a folder in your root joomla installation called 'dir', which you specified in your "Location of the Listing Folder". So, your files would be accessed via: http://yoursite.com/dir. Is that correct? But if that is the case, where does '/documenten/' come from?
By the way, do you have a web address of the page where this is on so that I can see it happening for myself?