I figured I'd do a quick blog on the error since it took me a little time to run down the problem when I received the following message running the VS 2010 debugger on the sandboxed appliction:
Error activating type from sandboxed code assembly. The cached sandboxed assembly could not be found.
I was working on as a SharePoint Sandboxed solution. The cause of the error is not so obvious. If your sandbox code depends upon assemblies in the GAC, and they are not in the GAC, then you will get this message. In my case I had an assembly in the GAC which depended upon an assembly that I had forgot to GAC. The error cleared once I added the second assembly to the GAC.