[Zope] Authentication Dialog comes again and again

Hi, I created a website with Zope that has its own acl_user folder with different users registered. When I log onto the site everything is fine, but by acessing certain pages Zope asks for a login again. Then I enter exact the same user and password and I can access the page. How can I prevent Zope from asking me for this useless login? Why does it come up? Thanks Eddy