I also did a fresh install of 1.3.1... getting the same error as James and from the same page.
I have some experience with various builds of PHP-Nuke and Kentico cms... so not a complete novice.
I found Oxwall during a search for a social networking software, figured I try something new, but still PHP.
I installed the software on one of my sub-domains created just to test it and immediately had an issue with the dreaded 500 internal error until I added "RewriteBase /" to the htaccess file.
The software then installed and seemed to run fine in every respect except the Main page causing these 404 errors.
~Merc