Omeka 2.3 with Ubuntu 15.04 - couldn't start and the errors.log is empty.

Have installed version omeka 2.3 in Ubuntu server 15.04, Apache 2.4 , MySQL 5.6.
Have change "AllowOverride None" to "AllowOverride All" in apache2.conf and change "log.errors = false" to "log.errors = true" in the file of errors.log.
Have also change the permissions for this file from none till writeable.
After restart the Apache, we start the omeka and we receive the sign "Omeka has encountered an error". The errors.log is empty. What to do ?

First step is to follow the steps here to turn on error messages and logging.