AppDomain.Load() is only meant to be called on AppDomain.CurrentDomain. (It's meant for interop callers only. They need a non-static method, and Assembly.Load() is static.) If you call it on a different AppDomain, if the assembly successfully loads in the target appdomain, remoting will then try to load it in the calling appdomain, potentially causing a FileNotFoundException/SerializationException for you.
If you need to execute an exe, use AppDomain.ExecuteAssembly() or (starting in v2.0) AppDomain.ExecuteAssemblyByName() instead. Otherwise, you should change to use Assembly.Load() from within the target appdomain. See Executing Code in Another AppDomain for more info.