"Comments" menu button on the left side of admin dashboard disappeared

After uploading the object-cache.php file to wp-content folder the apc caching works fine.

But there’s an issue: the “comments” button on the left side of the admin dashboard, aswell as the “contact form 7” menu button disappear after uploading the file and the files which these button point to (“edit-comments.php” for the comments button) are not accessible anymore, even not by navigating there directly through the browser’s address bar.

Even weirder is that an exact copy of the site and database on a testsite (same server etc.) doesn’t have this issue.

Anyone any idea what could be causing this?

Thanks for any reply in advance.

http://wordpress.org/plugins/apc/