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If you have retrieved the latest source code and build that, you can use the new Trace feature (refer to app.config in Interop). Create a config file next to the host .exe and give it the exact same name with an added .config postfix (like vsthost.exe.config).
Replace the [PluginName] placeholder with the name of your plugin (assembly file name - without extensions). You should be able to see what methods are called in the interop layer and what return values we provided by the plugin. Perhaps that will bring you
I've recently added deprecated member support but that has not been tested yet. So its possible a bug is doing this.
TIP: Make sure (by debugging) the WantMidi() method is called on the Host (deprecated) interface in the implementation of Resume of the plugin root class (Framework.Plugin).
I'm a little short on time right now but let me know how you do and I will try to aid you as best as I can (and time permits).