We have been using the UltraWebTab to contain two different user controls that users can select from. It has been working fine for many months and now suddenly starts failing repeatedly with errors like the one below but only when the server is busy. At low volume, I cannot reproduce this at all. We also have two different ways to long into the application and one way seems more susceptible to this issue than the other. Can anyone suggest what might be going on? This is not a trappable error so it is proving difficult to debug.

Exception Type:        System.NullReferenceException
Exception Message:     Object reference not set to an instance of an object.
Exception Source:      Infragistics35.WebUI.Shared.v9.2
Exception Target Site: GetPageContainer

---- Stack Trace ----
   Infragistics.WebUI.Shared.GlobalCssHelper.GetPageContainer(page As Page, create As Boolean)
       (unknown file): N 00404
   Infragistics.WebUI.Shared.GlobalCssHelper.OnInit(control As WebControl, page As Page)
       (unknown file): N 00082
   Infragistics.WebUI.UltraWebTab.UltraWebTab.OnPreRender(e As EventArgs)
       (unknown file): N 00149
   System.Web.UI.Control.PreRenderRecursiveInternal()
       (unknown file): N 00080
   System.Web.UI.Control.PreRenderRecursiveInternal()
       (unknown file): N 00171
   System.Web.UI.Control.PreRenderRecursiveInternal()
       (unknown file): N 00171
   System.Web.UI.Control.PreRenderRecursiveInternal()
       (unknown file): N 00171
   System.Web.UI.Control.PreRenderRecursiveInternal()
       (unknown file): N 00171
   System.Web.UI.Control.PreRenderRecursiveInternal()
       (unknown file): N 00171
   System.Web.UI.Page.ProcessRequestMain(includeStagesBeforeAsyncPoint As Boolean, includeStagesAfterAsyncPoint As Boolean)
       (unknown file): N 00842