I am not sure what the technical explaination is but
it is somthing like the configuration app is not actually
loading the entire plugin for the configuration, and the
configuration app will only look for assemblies in the
root MP folder so when you call your EPGfr.Serializers
MP cannot find it because it is not in the root folder.
What I did was created a MyPluginHelper.dll library to put my
serialization code (and other code) in and reference this
Now put the MyPlugin.dll in the correct plugin folder
and the MyPluginHelper.dll in the root MP folder and you
are up and running again.
Oh and don't worry about trying to handle AppDomain.CurrentDomain.AssemblyResolve
to point to your dll because the configuration already
does this and creates the messagebox you showed
above and exits the app.