Tiki doesnt log, but you can check the PHP error and access logs. Also make sure the postgres module is good. and remember postgres support is alpha, it could be that.
Also check things like Safe mode, open basedir, permissions, did you run ./setup.sh and has it created the necessary directories and permissions. Also what is your session.save_path set to in php.ini does it exist, is PHP allowed to write there? etc etc etc