I'm thinking of your site... when you go to login, do you go to
www.yoursite.com or just to yoursite.com? And then when you click on a link on your site, does it go to
www.yoursite.com or to yoursite.com (you can change that at vboptions > Site Name / URL / Contact Details > Site URL)? It used to be (and could still be if you modified your cookie settings), that if you logged in at
www.yoursite.com and the links took you to yoursite.com, then you would have the problem you are seeing.