sounds like they may be going to:
http://yoursite.tld/

When you have your full url set to:
http://www.yoursite.tld/

And running into a cross domain cookie issue. theres a solution for apache users in the faq forum that show you how to only allow one domain to access your site (and redirect the other)

I am a Web Development Contractor, I do not work for UBBCentral. I have provided free User to User Support since the beginning of these support forums.
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