[Zope] Zope Team(support required)

Hi ZopeTeam, We are using zope. We are facing one problem. Whenever we want to logoff from the zope user interface, I have choosed "logout" option and clicked the "Go" button in top of frame then it is showing username and password window. Why it is showing username and password window again?. Once i click the "OK" button it is logging off. So, anybody is give a solution "How to avoid a username, password window whenever we logging off?". Regards, M. Satish