an urgent problem on servlet and iPlanet Application Server?

[freeever]

I have an Web application running well on Tomcat,Weblogic and WebSphere. When I run it on iPlanet Application Server 6.0SP3 after having deploying it successfully, I got the following error message:
[05/June/2002 15:00:31:1] error: Exception: SERVLET-execution_failed: Error in e
xecuting servlet UIWebListener: java.lang.NullPointerException
Exception Stack Trace:
java.lang.NullPointerException
at com.netscape.server.servlet.servletrunner.ServletInfo.checkAuthorization(Unknown Source)
at com.netscape.server.servlet.servletrunner.ServletInfo.checkAuthorization(Unknown Source)
at com.netscape.server.servlet.servletrunner.ServletRunner.execute(Unkno
wn Source)
at com.kivasoft.applogic.AppLogic.execute(Unknown Source)
at com.kivasoft.applogic.AppLogic.execute(Unknown Source)
at com.kivasoft.thread.ThreadBasic.run(Native Method)
at java.lang.Thread.run(Thread.java:479)
[05/June/2002 15:00:31:1] error: APPLOGIC-caught_exception: Caught Exception:
Would you be kind enough to tell me the reason? Thanks a lot!