I have problems with my surveys because appears the message "expiration session". Well I modify the parameters of the server in order to get 24h to expiration time (87600) and I modify the config-defaults-php with the same value but It is not working, It happen the same error.
You should set a session save path - none is set.
Also you should check for other (PHP) applications that are on the same server that might wipe your session regularly. Last but not least you could try switching to database-based sessions editing by editing config.php.