Error enabling multisite network

Hi,

I already created several wordpress multisite networks, but experiencing a problem right now.

I used fresh files, downloaded from wordpress.org and uploaded and installed on my website. It't all working well... until the 1st step on network activation.

On the exact moment I insert...

define('WP_ALLOW_MULTISITE', true);

...on the wp-config, I refresh the admin panel and start to receive errors:

Warning: Cannot modify header information - headers already sent by (output started at /home/infoprodutofacil/public_html/wp-config.php:1) in /home/infoprodutofacil/public_html/wp-includes/option.php on line 787

Warning: Cannot modify header information - headers already sent by (output started at /home/infoprodutofacil/public_html/wp-config.php:1) in /home/infoprodutofacil/public_html/wp-includes/option.php on line 788

It's a fresh installation. Nothing was done on it. Just instalation and them the wp-config change...

(details removed by moderator, due to these being public forums)

Same user and password to access wp-admin.