OpenDocument not using existing session

Hi all,

I've created a dashboard in Dashboards 4.1 SP 4 and published it to BI Platform. There is a URL button in the dashboard linking to a WebI report in the same server.

According to what I 've read in SCN posts, if the dashboard is opened from within the BI launchpad, Open Document is supposed to use the existing session username and password but in my case , when I click the URL button in the dashboard to open a WebI report , a new tab is opened and the logon screen appears.

Am I missing something?

Thanks