I've sometimes seen this when you log in on different subdomains. Depending on the browser, the cookie from the subdomain is read before or after the cookie from the base domain and it gets confused. It will see the cookie and get a user_id, but it won't match the system and then bump the user out to the login page only to read the cookie again.
It's an older system I see this on, and also it was an issue we had from the dotcom days a lot (netscape and IE worked differently in terms of the precedence of readin ghte ocokie) and I haven't investigated fully in the current OpenACS....
In any case, if this is the problem, you can go to your browser options and clear your cookies and then try to log in again.
