Blogs.dir access problem

Whenever I try to upload any media files (Picture, video...etc) to any of the blogs created under our WP MU installation, I get the following error:

Unable to create directory /httpdocs/wp-content/blogs.dir/2/files/2010/11. Is its parent directory writable by the server?

I've tried to chmod blog.dir to 755, but it didn't work

I've tried to work on the ownership of the blog.dir based on the thread I've read here before
https://premium.wpmudev.org/forums/topic/new-blogs-get-error-message-when-uploading-media

But the only user on the server is the 'root' user so using chown doesn't help in any way...

So any suggestion how I can overcome this problem?

Thanks in advance for your time

*** Wanted to add that I've checked PHP safe mode and it is off