Are you getting this exact same error with the same stack trace that Koe posted or just a similar error?
What version of mojoPortal are you using?
Have you tried having the user clear all browser cookies and login again?
There was a different error related to the role cookie reported recently on this thread that appeared to be related to a recent security update for ASP.NET, and as mentioned on that thread I patched the 126.96.36.199 release to handle the error.
Hope that helps,