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
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