Have you configured the "back to home page" URL in the Admin Console (the "Website URL" line)? That should point to the referring web page that you have JForum linked on. Also, don't forget the trailing "/" on the "Forum URL" too.
I'm running a similar config as you, but on Hardened Gentoo,
Tomcat 5.0.28, and Apache 2 with mod_jk and I don't see this type of error. I'm using JForum 2.1.6 with CVS updates as of Sept. 9.
[originally posted on jforum.net by GatorBait3]