Java applet fails to start, gives NPE from IExplorerPluginObject$2.run() and hangs

OS: Windows XP(Japanese)
Java: 1.5.0_16

Infrequently Java plugin cannot load applet.
At that time, following messages are logged.
It shows NullPointerException happens at sun.plugin.viewer.IExplorerPluginObject$2.run.

<record>
<date>2010-01-12T17:49:50</date>
<millis>1263286190829</millis>
<sequence>84</sequence>
<logger>sun.plugin</logger>
<level>FINE</level>
<class>com.sun.deploy.util.LoggerTraceListener</class>
<method>print</method>
<thread>10</thread>
<message>java.lang.NullPointerException
</message>
</record>
<record>
<date>2010-01-12T17:49:50</date>
<millis>1263286190829</millis>
<sequence>85</sequence>a
<logger>sun.plugin</logger>
<level>FINE</level>
<class>com.sun.deploy.util.LoggerTraceListener</class>
<method>print</method>
<thread>10</thread>
<message> at sun.plugin.viewer.IExplorerPluginObject$2.run(Unknown Source)
</message>