You say your domain does NOT use www, but you link to a WWW version... I think this is your issue...

www.yoursite.tld and yoursite.tld are two totally diffeant subdomains; (one as a subdomain, one as no subdomain); this creates cross-cookie login issues when users try to use one and are redirected to the other by your "full url" setting in the UBB.

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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