Message about Uploads Folder

I have a worpdess multi site installation.

Yesterday, I upgraded to WP version 3.5 and upgraded all the sites in the network.

Today I created a new sub-site and I get the following error message on every page of the dashboard for this new sub-site:

"Upload folder is not writable. Export and file upload features will not be functional."

I don't have this message on the old sub-sites, just on this new one.

Please help