I've just upgraded a v12 tikiwiki to v14. I unpacked the v14 files into a clean folder, and ran the installer to upgrade the database. It had 7 issues, but nothing serious.
As soon as the installer finished though, tikiwiki now only responds with 500 errors. The PHP log states:
PHP Fatal error: Uncaught --> Smarty: Unable to read template file 'layout_view.tpl'