Have you installed any extra modules or anything? Sounds like you've somehow got two of something in the session that there's only meant to be one
![happy](/Modules/YetAnotherForum/images/emoticons/smile001.gif)
Failing this, try going to web.config in the Cuyahoga.Web folder and change the machinekey - just replace the first letter with any other character 0-9 or A-F.
What that will do is invalidate the old .NET session which should create a new one, hopefully without the junk. That's worked for me with similar weirdness in the past