TFS workItem.Validate() issue - tfs

Question

Here is the case. I wrote a tool to create a task WorkItem (version 12.0.0.0), it can run normally and create TFS task last month. But Today, when I rerun it, it doesn't create TFS task and show error message: The field "Assigned To" contains the value "email address" that is not in the list of supported values.
NetworkCredential credential = new NetworkCredential("user", "password");
TfsConfigurationServer configurationServer = new TfsConfigurationServer(tfsUri, credential);
ReadOnlyCollection<CatalogNode> collectionNodes = configurationServer.CatalogNode.QueryChildren(new[] { CatalogResourceTypes.ProjectCollection }, false, CatalogQueryOptions.None);
foreach (CatalogNode collectionNode in collectionNodes)
{
Guid collectionId = new Guid(collectionNode.Resource.Properties["InstanceId"]);
TfsTeamProjectCollection teamProjectCollection = configurationServer.GetTeamProjectCollection(collectionId);
ReadOnlyCollection<CatalogNode> projectNodes = collectionNode.QueryChildren(new[] { CatalogResourceTypes.TeamProject }, false, CatalogQueryOptions.None);
foreach (CatalogNode projectNode in projectNodes)
{
Console.WriteLine("Team Project: " + projectNode.Resource.DisplayName);
}
WorkItemStore workstore = teamProjectCollection.GetService<WorkItemStore>();
Project project = workstore.Projects["SpecifiedProject"];
WorkItemType itemtype = project.WorkItemTypes["Task"];
WorkItem workItem = new WorkItem(itemtype);
string assignedTo;
assignedTo = String.Format("{0}#***.com", task.User);
workItem.Fields["Assigned to"].Value = assignedTo;
workItem.Fields["Priority"].Value = task.Priority;
workItem.Fields["Area Path"].Value = AREAPATH;
workItem.Fields["Iteration path"].Value = task.IterationPath;
workItem.Title = task.Title;
workItem.Fields["DESCRIPTION"].Value = task.Description;
workItem.Fields["Original Estimate"].Value = task.EstimatedTime.ToString();
workItem.Fields["Completed Work"].Value = task.ActualTime.ToString();
workItem.Fields["Remaining Work"].Value = task.RemainTime.ToString();
// get the link type for hierarchical relationships
var linkType = workstore.WorkItemLinkTypes[CoreLinkTypeReferenceNames.Hierarchy];
//var linkType = workstore.WorkItemLinkTypes[CoreLinkTypeReferenceNames.Dependency];
RelatedLink rl = new RelatedLink(linkType.ReverseEnd, task.UserStoryID);
workItem.Links.Add(rl);
ArrayList ValidationResult = workItem.Validate();
I think this code block "ArrayList ValidationResult = workItem.Validate();" has some issues, for detail error: enter image description here