This week's book giveaway is in the OCAJP 8 forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide and have Edward Finegan & Robert Liguori on-line! See this thread for details.
I also encountered this issue in my website, I found that it's caused by the InstallServlet, if you access the install url in the installed forum, then the issue will be reproducible. In my website, I didn't remove the InstallServlet, sometimes maybe the install url access by search engines, then I got the error. So just remove the InstallServlet after installed jforum. Hope my message can help if someone got the same issue.