Working on an XMA lately I ran into an issue trying to use an object with .NET Remoting hosted on IIS7 (Windows Server 2008 x64). Turns out that IIS7 has a feature called Kernel Mode Authentication.
When .NET Remoting is configured to Negotiate the authentication type, it caused an error with the XMA. Turning off Kernel Mode Authentication fixed the problem.
Changing the authentication type probably would have worked too.