Import Error

I have no idea what this means, or what I have to do to fix it. I am setting up a website, including a blog, for my wife, and I am trying to transfer her posts to the new domain. I exported from her WP.com domain, but during the import to her own domain I get this:

The uploaded file could not be moved to
/folder

Warning: Cannot modify header information – headers already sent by

(output started at /folder/wordpress/wp-admin/admin-header.php:16) in

/folder/wp-includes/functions.php on line 1219