After successfully running TikiWiki 2.4 for 6 months, I decided to upgrade to 3.1.
I uploaded the tar.gz to my webhost where I have root privileges on a virtual host. I untarred 3.1 and then copied all files in the 3.1 directory to the old directory (tikidir).
I then ran .setup.sh (username) (groupname), deleted the "lock" file in the db directory and ran tiki-install.php. This proceeded successfully - php memory showing 32MB - and database connection validated. I then clicked on "Enter Tiki as admin and lock". From that point all I get is a blank screen, no matter what TikiWiki file I try to access.
Even when I try to access non-existent TikiWiki files, I still get just a blank screen. I get the blank screen in Mandriva Linux 2008.1 running Firefox and in Windows XP running IE.