When some were log in they get an error as below. all assistance appreciated. David

Stack Trace:

[NullReferenceException: Object reference not set to an instance of an object.]
   DotNetNuke.Security.Membership.AspNetMembershipProvider.UserLogin(Int32 portalId, String username, String password, String verificationCode, UserLoginStatus& loginStatus) +82
   DotNetNuke.Entities.Users.UserController.ValidateUser(Int32 portalId, String Username, String Password, String VerificationCode, String PortalName, String IP, UserLoginStatus& loginStatus) +41
   DotNetNuke.Modules.Admin.Security.Signin.cmdLogin_Click(Object sender, EventArgs e) +228
   System.Web.UI.WebControls.Button.OnClick(EventArgs e) +105
   System.Web.UI.WebControls.Button.RaisePostBackEvent(String eventArgument) +107
   System.Web.UI.WebControls.Button.System.Web.UI.IPostBackEventHandler.RaisePostBackEvent(String eventArgument) +7
   System.Web.UI.Page.RaisePostBackEvent(IPostBackEventHandler sourceControl, String eventArgument) +11
   System.Web.UI.Page.RaisePostBackEvent(NameValueCollection postData) +33
   System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +5102

[PageLoadException: Object reference not set to an instance of an object.]
   DotNetNuke.Services.Exceptions.Exceptions.ProcessPageLoadException(Exception exc, String URL) +351
   DotNetNuke.Framework.PageBase.Page_Error(Object Source, EventArgs e) +377
   System.Web.UI.TemplateControl.OnError(EventArgs e) +105
   System.Web.UI.Page.HandleError(Exception e) +73
   System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +6944
   System.Web.UI.Page.ProcessRequest(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +154
   System.Web.UI.Page.ProcessRequest() +86
   System.Web.UI.Page.ProcessRequestWithNoAssert(HttpContext context) +18
   System.Web.UI.Page.ProcessRequest(HttpContext context) +49
   ASP.default_aspx.ProcessRequest(HttpContext context) +4
   System.Web.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute() +154
   System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously) +64

Just a quick note for all: I found the error! It was a database mismatch: The record with the portal user-id existed in the aspnet_Users and aspnet_Membership tables, but the applicationID field did not match. Correcting the field by means of the SQL Server Enterprise Manager (copied the applicatioID GUID from aspnet_Applications) solved the problem.

Best regards,
Henning