During a deployment of a custom workflow to a CRM 2011 server I started to encounter errors in the plugin registration process. The Workflow Assembly would register with CRM but the Workflow activity registration would fail. I would get the following error

Unhandled Exception: System.ServiceModel.FaultException`1[[Microsoft.Xrm.Sdk.OrganizationServiceFault, Microsoft.Xrm.Sdk, Version=5.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35]]: Exception retrieving custom activity info - Could not load file or assembly 'Microsoft.Crm, Version=5.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35' or one of its dependencies. Access is denied.

The CRM server I was deploying to was shared with a number of other developers and this particular server had been experiencing other issues during the day. Once the server had been stabilised and the organisations were working once more I found I was still encountering the same issues. I was using the Plugin registration tool, was connected as the CRM Administrator to perform my deployments but it was still telling me that my access was denied.

It turns out that the Server/bin folder on the CRM server was missing some permissions which was why I was getting the Access Denied message. In particular, the CRMServer\User group was missing and once I had reinstated it my Workflows would register the assembly and custom activities successfully.