This is a different problem than you had: you couldn't log out. That, I found out, was caused by a duplicate login cookie (no idea how that happens)
This problem (half of pages redirecting you to logon) are a result of the horrible frankenstein that is the combination of .NET and the old ASP code: basically, the old pages that are wrapped in .NET forgot that you are logged in.
This happens to me on CYS every once in a while; just wish I knew why.