Help? I can't edit my workflows!!

Hi

I am trying to edit an existing workflow, but whenever I unpublish it I get the "Consult your CRM Administrator" error message.

Here is the trace log, and it seems to be a problem with a Custom Workflow Activity, but I can't see what the issue is?

[2013-03-05 16:27:41.6] Process: w3wp |Organization:76c6b677-a2ea-43e0-a88c-cf2358b17a33 |Thread:   24 |Category: Application |User: 00000000-0000-0000-0000-000000000000 |Level: Error | ErrorInformation.LogError
>MSCRM Error Report:
--------------------------------------------------------------------------------------------------------
Error: Exception of type 'System.Web.HttpUnhandledException' was thrown.

Error Message: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.

Error Details: Exception of type 'System.Web.HttpUnhandledException' was thrown.

Source File: Not available

Line Number: Not available

Request URL: http://sauron/BERT/sfa/workflow/edit.aspx?id={6CDEC756-EFD8-4715-93BA-E6D07865874E}

Stack Trace Info: [NullReferenceException: Object reference not set to an instance of an object.]
   at Microsoft.Crm.Application.Platform.CustomActivityInfoMetadata.GetActivityInfo(Guid pluginTypeId)
   at Microsoft.Crm.Web.SFA.WorkflowDetailPage.ConfigureCustomActivityStepMenu(CommandBarPopup addStepButton)
   at Microsoft.Crm.Web.SFA.WorkflowDetailPage.ConfigureWorkflowStepMenu(WorkflowStep workflowStep)
   at Microsoft.Crm.Web.SFA.WorkflowDetailPage.ConfigureMenus()
   at Microsoft.Crm.Application.Controls.AppUIPage.OnPreRender(EventArgs e)
   at System.Web.UI.Control.PreRenderRecursiveInternal()
   at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)

[HttpUnhandledException: Exception of type 'System.Web.HttpUnhandledException' was thrown.]
   at System.Web.UI.Page.HandleError(Exception e)
   at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)
   at System.Web.UI.Page.ProcessRequest(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)
   at System.Web.UI.Page.ProcessRequest()
   at System.Web.UI.Page.ProcessRequestWithNoAssert(HttpContext context)
   at System.Web.UI.Page.ProcessRequest(HttpContext context)
   at ASP.bert_sfa_workflow_edit_aspx.ProcessRequest(HttpContext context)
   at System.Web.HttpApplication.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute()
   at System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously)

Any help would be much appreciated

Andy