Problems with uploading images (image path) on network sites

I have just followed the manual found here to set up a WordPress MU site. Everything runs fine except from image uploading. The files are uploaded to e.g. "/wp-content/blogs.dir/2/files/2012/12/logo.png" and I can see them on the webserver with my FTP program, but in the media library in the WP Dashboard the image path is reported to be "/site-title/files/2012/12/logo.png".

What is my problem, and how can I solve it??