When I click on "delete cache" in tab contents, the cache is not deleted. All files in the wp-content/cache/supercache folder are still there.
On the other hand when enabling and disabling the caching, the folder-content is deleted.
And also if I save the settings of the WP SEO Plugin by Yoast, the cache is deleted correctly.
So nearly all ways work, but not the direct delete cache link on the contents tab.