Any idea as to why I would get this error? I searched the forum and only found one other post and I'm not sure that the previous thread applies to me.

I don't think you got it replaced then.  I am pretty confident that the issue has been resolved and the behavior you are getting is indicitive of a version conflict.  I went through this a number of times before I posted the fix.  Are you sure that you updated the GAC?  Try manually deleted the Extensibility DLL in the GAC and the adding it back with the new one.