Likely the user is going to your site as (and logging in to)
http://yoursite.tld

Once they login, they're redirected to your "full url" you set in your CP; which forces them to have to login again, as the "full url" (http://www.yoursite.tld) is seen as a different domain than http://yoursite.tld and thus the cookie is invalid...

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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