[plug-in] WP super cache

Hi,

I got the following error when I try to change wp-super-cache from off to on or the half-on:
Warning: rename(/home/netsavage/public_html/blog/wp-content/cache/supercache/www.netsavage.com,/home/netsavage/public_html/blog/wp-content/cache/supercache/www.netsavage.com.disabled) [function.rename]: Permission denied in /home/netsavage/public_html/blog/wp-content/plugins/wp-super-cache/wp-cache.php on line 684

So doe I have to chmod some files different?
Sometimes the cached page shows a blank screen instead of the post so I deactivated the plugin for now. Any ideas on that?

wp=2.6

thx!