Looks like a dynamically generated dll. If you use e.g. XMLSerializer, such dlls are created for each type to be serialized/deserialized, dynamically at run-time, in the temp folder. And that's the point where something went wrong with your service.
What does happen when you debug the service from Visual Studio (note: you must use some tricks to do so, e.g.
Debugging Windows Services under Visual Studio .NET[
^])?
You could write your own code for serialization/deserialization of the objects to prevent that problem.