IE9 shows JS error in the custom widgetset

We have Vaadin 6.7.4 portlets running in Liferay portal 6.1. For our application, we have created custom widgetsets and compile them using GWT 2.3.0. But the page shows JS error in some of the newer updated versions of IE9. The developer tool shows JS error, and the page shows the spinning wheel as if its waiting for a response. The error that I see is

SCRIPT16389: error -2147467259
5EBBFBEFFBAD31F6B3CCE4D232979138.cache.html, line 1 character 1

On running the application in debug mode, I also see

JavaScriptException: (ReferenceError): ‘uIb’ is undefined
description: ‘uIb’ is undefined
number: -2146823279

The issue is only specific to IE9 newer updated versions. Chrome, FF, Safari all works fine.

Also this issue occurs only on portlets using our custom widgetset. Liferay comes with a PortalDefaultWidgetset, pages using that widgetset renders fine. Is there any components used in custom widgetset could be causing this? If yes, how do I troubleshoot it?

Can anyone help me resolve this issue?

Thanks
Ratheesh

Any help on this? After running the vaadin application in debug mode, I see this

SCRIPT16389: error -2147467259
5EBBFBEFFBAD31F6B3CCE4D232979138.cache.html, line 1 character 1
LOG: Starting Vaadin client side engine. Widgetset: com.web.widgetset.CommonWidgetset
LOG: Widget set is built on version: 6.7.4
LOG: Starting application v-interstitial_WAR_interstitialportlet_INSTANCE_fJa0_LAYOUT_310801
LOG: Vaadin application servlet version: 6.7.4
LOG: Application version: NONVERSIONED
LOG: inserting load indicator
LOG: setShowMarker:startRequest
LOG: Show marker:true markerCount:1
LOG: create Maske Element:=null
LOG: maskElement:=[object HTMLDivElement]

LOG: Making UIDL Request with params: init
LOG: Server visit took 48ms
LOG: JSON parsing took 0ms
JavaScriptException: (ReferenceError): ‘uIb’ is undefined
description: ‘uIb’ is undefined
number: -2146823279
LOG: inserting my own load indicator
LOG: indicator left :=776 top:=162

Thanks
Ratheesh