Microsoft.SharePoint.SPException: The security validation for this page is invalid

Question

I used SharePoint Designer 2007 to make a Workflow that copy item of Calendar list to Contact list when I create a new item. Then I run my code that new item in Calendar list. Unfortunately, I get a message as below

Microsoft.SharePoint.SPException: The security validation for this page is invalid. 
Click Back in your Web browser, refresh the page, and try your operation again. --->     
System.Runtime.InteropServices.COMException (0x8102006D): The security validation for this page is invalid. Click Back in your Web browser, refresh the page, and try your operation again.       
at Microsoft.SharePoint.Library.SPRequestInternalClass.ValidateFormDigest(String bstrUrl, String bstrListName)   
at Microsoft.SharePoint.Library.SPRequest.ValidateFormDigest(String bstrUrl, String bstrListName)   
--- End of inner exception stack trace ---   
at Microsoft.SharePoint.Library.SPRequest.ValidateFormDigest(String bstrUrl, String bstrListName)   
at Microsoft.SharePoint.SPWeb.ValidateFormDigest()   
at Microsoft.SharePoint.Workflow.SPWorkflowManager.StartWorkflow(SPListItem item, SPWorkflowAssociation association, SPWorkflowEvent startEvent, Boolean bAutoStart, Boolean bCreateOnly)   
at Microsoft.SharePoint.Workflow.SPWorkflowManager.StartWorkflow(SPListItem item, SPWorkflowAssociation association, String eventData, Boolean isAutoStart)   
at Microsoft.SharePoint.Workflow.SPWorkflowManager.StartWorkflow(SPListItem item, SPWorkflowAssociation association, String eventData)   
at StartWorkflowHelper.StartWorkflowFunction.StartWorkflow(SPListItem item)   
at AddItem.btnSubmit_Click(Object sender, EventArgs e)

And my code to run workflow :

//Class Handle Workflow and I built this class to dll and set it into GAC.           
     namespace StartWorkflowHelper
        {
            public class StartWorkflowFunction
            {
            [System.Security.Permissions.PermissionSet (System.Security.Permissions.SecurityAction.Assert, Unrestricted = true)]                
            public static void StartWorkflow(SPListItem item)
            {
                SPList list = item.ParentList;
                SPEventManagerWrapper.DisableEventFiring();
                SPWorkflowAssociationCollection wfac = list.WorkflowAssociations;
                using (SPWorkflowManager manager = item.Web.Site.WorkflowManager)
                {
                    foreach (SPWorkflowAssociation wfa in wfac)
                    {
                        if (wfa.AutoStartChange || wfa.AutoStartCreate)
                        {
                            manager.StartWorkflow(item, wfa, "");
                        }
                    }
                }
                SPEventManagerWrapper.EnableEventFiring();
            }            
        }

        public static class SPEventManagerWrapper
        {
            private static readonly string _className = "Microsoft.SharePoint.SPEventManager";

            private static readonly string _eventFiringSwitchName = "EventFiringDisabled";

            private static Type _eventManagerType;

            //Gets the status of event firing on the current thread
            public static bool EventFiringDisabled
            {
                get { return GetEventFiringSwitchValue(); }
            }

            private static Type EventManagerType
            {
                get
                {
                    if (_eventManagerType == null)
                        GetEventManagerType();
                    return _eventManagerType;
                }
            }

            //Enables event firing on the current thread
            public static void EnableEventFiring()
            {
                SetEventFiringSwitch(false);
            }

            //Disables sharepoint event firing on the current thread
            public static void DisableEventFiring()
            {
                SetEventFiringSwitch(true);
            }

            //Sets the event firing switch on Microsoft.SharePoint.SPEventManager class using reflection
            private static void SetEventFiringSwitch(bool value)
            {
                PropertyInfo pi = EventManagerType.GetProperty(_eventFiringSwitchName, System.Reflection.BindingFlags.Static | System.Reflection.BindingFlags.NonPublic);        
                pi.SetValue(null, value, null);
            }

            private static bool GetEventFiringSwitchValue()
            {
                PropertyInfo pi = EventManagerType.GetProperty(_eventFiringSwitchName, System.Reflection.BindingFlags.Static | System.Reflection.BindingFlags.NonPublic);        
                object val = pi.GetValue(null, null);        
                return (bool)val;
            }

            private static Type GetEventManagerType()
            {
                _eventManagerType = typeof(SPList).Assembly.GetType(_className, true);        
                return _eventManagerType;
            }
        }
    }

//Method Add New Item of Calendar list
....

    Bamboo.StartWorkflowHelper.StartWorkflowFunction.StartWorkflow(item); //item = SPListItem

Answer 1

EDIT: moved from comments

I am working at Bamboo. I had fixed this issue and I wanna share all. I checked sharepoint logs and found out a message "System.Reflection.TargetInvocationException: Exception has been thrown by the target of an invocation. ---> System.Security.SecurityException: That assembly does not allow partially trusted callers." To fix this message, you insert "Unrestricted=true" property.