Problem with wp-config.php when runing network

Hi all,

I am trying to install a network, but still have the same issue again and again (after clean several times the database and made fresh installs many times).
I can install WP without any problem (for single site), when I add to wp-config file the instruction to allow multisite, it works and the “network” menu appears.
After that I change into wp-config the values as indicated by the system, I changed the .htaccess too. At this moment, I can access to the website, but it’s impossible to access to wp-admin section.
I change debug value to true, and this is the message I have :
Warning: Cannot modify header information – headers already sent by (output started at my_server_folder/wp-config.php:1) inmy_server_folder/wp-includes/pluggable.php on line 1171

Someone would have an idea ?

Note, this is a fresh install. No exotic plugin or theme, nothing except the WP install files.

Thanks !