403 error with Bookmark this and Report this links

Hello all,

On my installs, when I click either the 'Bookmark this' or 'Report this' link in the left side menu, I receive a 403 Forbidden error. However, when I access the add.php file for either of these plugins manually, without passing a URL to it, the page returns fine.

Any ideas why these two links do not work correctly?

Many thanks,

Simon