Error unregistering 4.0 plugin after upgrade to 2011

Hello,

I’ve recently come across an issue where we had a third-party plugin that would not unregister using the registration tool or the web interface. The error message inner details are as follows:

System.IO.FileLoadException: Microsoft Dynamics CRM has experienced an error

After much trial and error, and with some help from Microsoft support we were able to determine that the server did not have Microsoft.Crm.Sdk.dll (x64) installed in the GAC. Now I won’t go into the obvious question of why it needs to have the old SDK dll in order to unregister a plugin (the new one is supposed to be backwards compatible) but once the assembly was registered and an iisreset run I was able to successfully remove it.

Hope that save someone else valuable time.