Recently I had a remarkable amount of trouble with the SDK on a particular CRM deployment. After a spot of poking around I managed to find that this deployment actually consists of 3 servers.
This configuration seemed to be working fine for some time with plugins hosted on the Application Server, but when I came to write applications using the web services hosted on the Platform Server I received the following error: 0x80044191 Assembly can not be loaded from C:\Program Files\Microsoft Dynamics CRM\server\bin\assembly\MyOrganisation.Plugins.dll. Platform.
Here's the fix:
If plugins are registered, ensure they are deployed with source on database.
If the plugin source MUST be on disk, then it must be stored on the Platform Server. Not the Application Server