sounds like a permission conflict with cookies. Ensure that the users are logging in from www.yourdomain.tld if you have www.yourdomain.tld set as your domain in the ubb config; what tends to happen is users try to login at yourdomain.tld instead and when redirected they're not logged in. There is a fix for apache servers listed in the FAQ forum (Redirect Non-WWW users to WWW)

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