Hmmm... that's odd. Not sure why a C# source file would be referenced at runtime. Does the name of that file look familiar to you (ie. any relationship to the assembly that's generated?)
But it looks like a permissions issue - .NET compiler trying to generate some code on the fly and probably not having rights to write to the temp folder. Try bumping the permissions in that path (if possible). I'm not sure why this should be happening for any of the generated assemblies though.
+++ Rick ---
from Hood River, Oregon