The root activity type is invalid

Assembly information for one or more assemblies referencing a type is missing from the safe list in the web.config file.
On the server, revert the web.config file in the root directory to the original version. This may require a clean install of Microsoft Office SharePoint Server 2007 or Microsoft Windows SharePoint Services 3.0.
(or)
Make sure that the following markup is present in your web.config file. You can copy the necessary sections or the entire markup as necessary.

<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="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="mscorlib, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089" Namespace="System" TypeName="Guid" Authorized="True" />
<authorizedType Assembly="mscorlib, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089" Namespace="System" TypeName="DateTime" Authorized="True" />
<authorizedType Assembly="mscorlib, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089" Namespace="System" TypeName="Boolean" Authorized="True" />
<authorizedType Assembly="mscorlib, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089" Namespace="System" TypeName="Double" Authorized="True" />
<authorizedType Assembly="mscorlib, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089" Namespace="System" TypeName="String" Authorized="True" />
<authorizedType Assembly="mscorlib, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089" Namespace="System.Collections" TypeName="Hashtable" Authorized="True" />
<authorizedType Assembly="mscorlib, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089" Namespace="System.Collections" TypeName="ArrayList" Authorized="True" />
<authorizedType Assembly="mscorlib, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089" Namespace="System.Diagnostics" TypeName="DebuggableAttribute" Authorized="True" />
<authorizedType Assembly="mscorlib, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089" Namespace="System.Runtime.CompilerServices" TypeName="CompilationRelaxationsAttribute" Authorized="True" />
<authorizedType Assembly="mscorlib, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089" Namespace="System.Runtime.CompilerServices" TypeName="RuntimeCompatibilityAttribute" Authorized="True" />
<authorizedType Assembly="mscorlib, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089" Namespace="System" TypeName="Int32" Authorized="True" />
<authorizedType Assembly="mscorlib, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089" Namespace="System" TypeName="TimeSpan" Authorized="True" />
<authorizedType Assembly="mscorlib, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089" Namespace="System.Collections.ObjectModel" TypeName="Collection`1" Authorized="True" />
<authorizedType Assembly="System.Workflow.Runtime, Version=3.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35" Namespace="System.Workflow.Runtime" TypeName="CorrelationToken" Authorized="True" />
it solved my problem…

hurry, happy coding…