WordPress MU

Hi.
This argument was already treated, but I can't find a solution.
When I try to load an image or another file, I have this error:
Impossibile creare la directory /home/web/www/blog/wp-content/blogs.dir/1/files/. Verifica che la directory madre sia scrivibile dal server!

Translate:
Unable to create directory / home/web/www/blog/wp-content/blogs.dir/1/files /. Is its parent directory writable by the server?

I have the permissions 777 in the folders "wp-content" and "blogs.dir".
I also created the folders "/1/" and "/files/", but nothing...

My WordpressMU is in a sub-domain (ex. wpmu.website.com).
Can you help me?
Thank you and sorry for my english.

The folder may not have the proper owner on them. Should be owned by the web account.

Thanks.
In the directory "blogs.dir" I couldn't change the permissions and so the permissions have been changed by my host.
Then, I created the folder /1/ and afterwards
the folder /files/
Now, it works!