SPD designer Workflow error. Failed to compile the workflow

Hello,

I am gettinhg this error when opening a workflow in SPD.

Errors were found when compiling the workflow.The workflow files were saved but cannot be run.

Furthermore when I am trying to click the advanced button on the error message I get the:
following message

(1, 1) The root activity type is invalid.)

Here is the ComponentModel.WorkflowCompiler section from the webconfig


 <System.Workflow.ComponentModel.WorkflowCompiler>
        <authorizedTypes>
            <authorizedType Assembly="System.Workflow.Activities, Version=3.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35" Namespace="System.Workflow.*" TypeName="*" Authorized="True"/>
            <authorizedType Assembly="System.Workflow.ComponentModel, Version=3.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35" Namespace="System.Workflow.*" TypeName="*" Authorized="True"/>
            <authorizedType Assembly="System.Workflow.Runtime, Version=3.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35" Namespace="System.Workflow.*" TypeName="*" Authorized="True"/>
            <authorizedType Assembly="System.Transactions, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089" Namespace="System*" TypeName="*" Authorized="True"/>
            <authorizedType Assembly="mscorlib, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089" Namespace="System*" TypeName="*" Authorized="True"/>
            <authorizedType Assembly="System, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089" Namespace="System*" TypeName="*" Authorized="True"/>
            <authorizedType Assembly="Microsoft.SharePoint, Version=12.0.0.0, Culture=neutral, PublicKeyToken=71e9bce111e9429c" Namespace="Microsoft.SharePoint.Workflow" TypeName="SPWorkflowActivationProperties" Authorized="True" />
            <authorizedType Assembly="Microsoft.SharePoint, Version=12.0.0.0, Culture=neutral, PublicKeyToken=71e9bce111e9429c" Namespace="Microsoft.SharePoint.Workflow" TypeName="SPWorkflowTaskProperties" Authorized="True" />
            <authorizedType Assembly="Microsoft.SharePoint, Version=12.0.0.0, Culture=neutral, PublicKeyToken=71e9bce111e9429c" Namespace="Microsoft.SharePoint.Workflow" TypeName="SPWorkflowHistoryEventType" Authorized="True" />
            <authorizedType Assembly="Microsoft.SharePoint.WorkflowActions, Version=12.0.0.0, Culture=neutral, PublicKeyToken=71e9bce111e9429c" Namespace="Microsoft.SharePoint.WorkflowActions" TypeName="*" Authorized="True" />
            <authorizedType Assembly="DP.Sharepoint.Workflow, Version=1.0.0.0, Culture=neutral, PublicKeyToken=0298457208daed83" Namespace="DP.Sharepoint.Workflow" TypeName="*" Authorized="True" />

        </authorizedTypes>
    </System.Workflow.ComponentModel.WorkflowCompiler>

Please suggest how to fix this.