I haven't worked with Kentico since v3.1. I have an install running in a virtual machine, it is configured to run in the URL localhost/KenticoCMS/. I created a page called forum, it renders at localhost/KenticoCMS/forum.aspx. I then added a Forum Group, added a forum, then configured the Forum Group webpart to utilize a Friendly URL scheme. (Web part properties >> General Tab >> Friendly URLs >> Use Friendly URLs.) Forum.aspx correctly shows the forum group, but when I try to click into my test forum (titled "Forum 01"), it links to /KenticoCMS/forum/f1/Forum-01.aspx which returns an error "The resource cannot be found".