OWA Sign Out doesn't work

I'm testing Exchange 2013 and running into the issue. On the OWA page, when I click on Sign out, OWA doesn't sign out.. It gives me "Sign Out" window with statement: "You must close all browser windows to sign out completely". Same behavior on local server, on remote server.. /OWA /ECP site.. Admin user, non admin user..

Forms authentication is disabled, basic and windows are enabled.

If I manually add /owa/logoff.owa it logs me off..

Any ideas?

Regards, Mindaugas Laucius