Hi,
Are you disabling caching when a cookie is detected? Set it to “Enable caching for all visitors” (or the last one, to disable it for logged in visitors only) instead. If the cache is disabled by the wp-discuz plugin, you should talk to the author of that plugin and show them this thread. They can add support for WP Super Cache using the following code.
You need to tell WP Super Cache to use those cookies to identify users because it only uses the default WordPress cookies by default. Read this page to learn more, but you should add the following lines to an mu-plugin file:
do_action( 'wpsc_add_cookie', 'comment_author_email_{HASH}' );
do_action( 'wpsc_add_cookie', 'comment_author_{HASH}' );
do_action( 'wpsc_add_cookie', 'comment_author_url_{HASH}' );
Replace the HASH at the end with whatever it is on your site, and test it. It should create a new cache file when you leave a comment.
Donncha