Yeah mdr1 has the same problem. I reported the issue via ourl.ca/issue (although it would be better if each user do it on their own in the future), because there could be many more users that can't login than we think, but won't necessarily tell us.
Someone recently said that this board uses the same cookie as the old board, so I think it's also a cookie conflict issue, so maybe admins will have to change its name. In SMF admin panel if you go to Config->Server settings->Cookies and sessions, there should be a cookie name like "SMFCookie11" (that's the one on TIMGUL archives), so if, for example, it's called SMFCookie533, it could be changed to SMFCookies472 and maybe it might solve the issue. The only downside is that this would result in every single user getting logged out and having to re-login again.