Iterate List after DelayActivity

Hello,

I have an workflow where I want to iterate a list after a delay.

SPWeb thisWeb = Context.Web;
SPList list = thisWeb.Lists["ListeExt"];
SPListItemCollection items = list.GetItems();
foreach (SPListItem item in items)
{
  .....
}

Where Context is the context obtain by applyActivation. First time if I iterate the List before the delay it works but after the delay get an error.

However thisWeb, list and items instances are not null. But immediately when I want to use the items object I have an error.

Further I can't see error in debug because after a delayActivity, visual Studio can't follow the execution...

I have look the log :

Area     : SharePoint Foundation
Category : Workflow Infrastructure
Level    : Unexpected
EventID  : 98d4
Message  : System.NullReferenceException: Object reference not set to an instance of an object.     at AlertesDRCP.Workflow_Alertes_DRCP.Workflow_Alertes_DRCP.checkAlertes_ExecuteCode(Object sender, EventArgs e)at System.Workflow.ComponentModel.Activity.RaiseEvent(DependencyProperty dependencyEvent, Object sender, EventArgs e)     at System.Workflow.Activities.CodeActivity.Execute(ActivityExecutionContext executionContext)     at System.Workflow.ComponentModel.ActivityExecutor`1.Execute(T activity, ActivityExecutionContext executionContext)at System.Workflow.ComponentModel.ActivityExecutor`1.Execute(Activity activity, ActivityExecutionContext executionContext)     at System.Workflow.ComponentModel.ActivityExecutorOperation.Run(IWorkflowCoreRuntime workflowCoreRuntime)     at System.Workflow.Runtime.Scheduler.Run()

But when I check item is not null...

Example this code has error :

if(items != null)
{
    int nb = items.Count;
}

EDIT:

I have forget to specify that my List is an External List. I have found an other error message like this :

Microsoft.SharePoint.SPException: Access denied by business Data Connectivity. ---> Access Denied for User '', which may be an impersonation by 'CFM\spfarm'. Securable MethodInstance with Name 'ReadList'denied access.

ideas ?

Thank you.