It happens even if you begin installing from scratch? If you can't get it working, you can always try manual installation:
http://www.jforum.net/install_no_wizard.jsp
However, JForum will not work inside the directory "jforum" - it's directories (templates, images etc) should be in the root directory.
Probably the next version (2.1.5) will allow this kind of personalization, but current (2.1.4 and older) do not.
Rafael
[originally posted on jforum.net by Rafael Steil]