First, are you sure that the include tag is the problem?
Have you tried taking it out to see if the exception is still being thrown?
If it is the problem, are you trying to forward or redirect from within the included page?
We have a FAQ entry with some of the most common causes of IllegalStateExceptions. You might find it helpful in your case.
You can read it here:
http://faq.javaranch.com/java/IllegalStateException