It's extremely hard to say anything definitive in such a broad issue scope. I'll try to give some suggestions for avenues of investigation (and some that I don't think are relevant).

• If RegistrationStatus.Pending has the int-value of say 0, or -1, or a similar possible also default value, some other problem such as a try-catch-all hiding some other problem might cause your GetRegistrationStatus to return the wrong result, which is subsequently interpreted as Pending. You should try to avoid using 0 as an actual value in enums for that reason.
• If you have recycling enabled on the application pool, you may get involuntary recycling, causing the session to be reset. The defaults are unfortunately recycling after 1740 minutes and 20 minutes of idle. I suggest turning both off. The symptoms are not quite right though, since that would probably cause a single redirect - unless they then bypass the code were Session["AttendeeId"] actually is set.
• Include some logging! The easy way is to use System.Diagnostics.Debug.WriteLine, you can watch the output from Visual Studio or use sysinternals DebugView. Better is to add some real logging using System.Diagnostics.Trace and freinds - or why not go all the way and add some HealthMonitoring! Specifically, you should break appart the one-liners and log the actual values received.

I don't think that it's a problem with cookies as such, since it appears to work out if you restart the browser. If cookies are disabled in the client it won't work at all, ever.

Thinking some more on it, I really think recycling might be the issue. You can check in the event log if recycling has occurred at about the same time where the users have experienced their problems.