Fatal error and i_am_super_user=salt

Hi all,

I tried to upgrade from piwik 2.16.1 to 3.0.4.
All was fine : upgrade command was succesfull.
But when I started piwik, the next message was displayed :

A fatal error occurred
Please contact the system administrator: or login to Piwik to learn more.
If you are Super User : but cannot login because ofthis error_ you can still troubleshoot funher.
Follow these steps:

  1. open the config/config.ini.php file and look for the value under (General)
  2. edit this current URL you are viewing and add the following text (replacing
    value from config by the value from theconfig file):
    Imlex.php?i_am_super_user= salt value

I followed the instructions, but unfortunately, it did’t work !
I had to restaure in 2.16.2.

Is it really a problem of authorization ? What is exactly the salt value ? how this value is generated in the config.ini.php file ? Is the value in rapport with the token auth value ? Any idea to resolve my problem, to succeed upgrade in 3.0.4 ?

Thanks for your help