HELP! - JSP/Struts/ADF and JBO-29000 exception

Question

What is the meaning of following exception:
Validation Error
You must correct the following error(s) before proceeding:
JBO-29000: Unexpected exception caught:
java.lang.NullPointerException, msg=null
null
I get this exception when the browser session is open for long period of time (I tried to time it but could not see consistent outcome. But usually it happens after about 1 hour).
And also the exception is thrown only for the JSP that was used (before timeout happened). I can still access other JSPs in the application that were not used earlier.
These random exceptions are making the whole application unusable. The user has to close the browser session and start over again.
Thaks in advance for the help.

Answer 1

Following are the settings for my application:
The session time out is 35 minutes (<session-timeout>35</session-timeout> in web.xml)
jbo.ampool.maxinactiveage is 600000 milliseconds (or 'Idle Instance Timeout' is 600 secs. or 10 mins.)
My application times out after 35 mins. and the user has to re-login (I handle this in my application by checking some session parameters that I set on login). The application works fine again except that sometimes I get the exception that I mentioned in this thread.