WordPress 3.5 changed the temp directory order to prefer the system’s pre-defined temp directory instead of attempting to write into wp-content. On some badly configured hosting systems, this temp directory may be defined, but not actually writeable. This causes the error message.
You can work around this by specifying a temp directory on your server with a place that you know WordPress is allowed to write files to. You can do this by adding this line of code into the wp-config.php file:
define( 'WP_TEMP_DIR' , ABSPATH . 'wp-content/' );