The quick fix is to delete or rename the install.jsp file and remove the install
servlet mapping from your web.xml.
The underlying issue seems to be that the InstallServlet will partially re-initialize the jForum environment in it's init method (well, in the class it's built on.). This is causing a lot of information that templates use to not be re-initialized. Most because the install servlet assumes that the db info is not available (since the app hasn't been installed).
The "right" way to fix it would probably be to modify the JForumBaseServlet.init method to set a ServletContext level flag that could be checked to see if an init was needed. Eg: add something like the code below to the init method.
[originally posted on jforum.net by monroe]