Element OBJUSER is undefined in a Java object

I have a app the creates a session scoped object (objUser) when a member signs in.

This object performs several task during the validation process making call similiar to:

session.objUser.setUpdateMembershipPaymentPermission(rc.objSecurityTopic.strPermission);
or
session.objUser.setSearchMembershipPackagePermission(rc.objSecurityTopic.strPermission);
etc

There are several such calls made during the process.

For some reason I can't explain every now and then one of them will throw:
Element OBJUSER is undefined in a Java object of type class [Ljava.lang.String;..

Why can't the CF server find session.objUser when just a millisecond before it did?

Thx

pwp