Java SE (Java Platform, Standard Edition)

Hi all,
I have applet for creating digital signatures of documents in browser. This applet runs without problems for about a year, but in last two or three weeks some users reports problems with them. After some testing I find that on some XP and Vista machines with IE 7 and JRE 1.6.0_05-b13 java throws this exception:

java.lang.NullPointerException: null pData
at sun.awt.windows.WComponentPeer.hide(Native Method)
at java.awt.Component.removeNotify(Unknown Source)
at java.awt.Container.removeNotify(Unknown Source)
at java.awt.Container.removeNotify(Unknown Source)
at java.awt.Container.removeAll(Unknown Source)
at sun.plugin.viewer.frame.IExplorerEmbeddedFrame.windowClosed(Unknown Source)
at java.awt.Window.processWindowEvent(Unknown Source)
at java.awt.Window.processEvent(Unknown Source)
at java.awt.Component.dispatchEventImpl(Unknown Source)
at java.awt.Container.dispatchEventImpl(Unknown Source)
at java.awt.Window.dispatchEventImpl(Unknown Source)
at java.awt.Component.dispatchEvent(Unknown Source)
at java.awt.EventQueue.dispatchEvent(Unknown Source)
at java.awt.EventDispatchThread.pumpOneEventForFilters(Unknown Source)
at java.awt.EventDispatchThread.pumpEventsForFilter(Unknown Source)
at java.awt.EventDispatchThread.pumpEventsForHierarchy(Unknown Source)
at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
at java.awt.EventDispatchThread.run(Unknown Source)

Applet always finish his work, write results into hidden form fields on the page and after sending the form to server and reloading page at client this exception is thrown.
Does anybody has similar problems? Or some tips why the problems started in recent weeks (some windows/IE patches or something else)?
Thanks a lot,

Zdenek Drlik