In the Sibsoft Client Area  (your account) you will see that, your website is normally registered as 
http://domain.com   (this means there is no 
www.)
This also means the desktop upload only works if the domain does NOT contain "www".
- The desktop uploader grabs URL from client area & does not follow redirects.
 
So you should not add those .htaccess code and the desktop uploader will work.