First, the warning is there for the default "podcast" channel. I've added a note that we should allow this note/warning to be disabled for situations like this.
It appears something is conflicting with the feed. Try disabling plugins one by one until you can determine which plugin is causing the conflict. If it is not a plugin, then try switching to the stock theme that is packaged with WordPress (twentyeleven/twentyten).
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If you don't mind, please reply with the URL to your custom post feed. If this doesn't exist, this may indicate something as simple as the custom post type not setup as type "post". Confusing I know, but if the custom post type is set to something custom or something like "pages", then that would explain the empty feed.