Cannot modify header information – headers already sent by (wp-config.php:1)

So yesterday i upgraded from CentOS 6.6 to CentOS 6.7 and rebooted my server.

After a reboot the following error messages with a blank page appeared:

Warning: Cannot modify header information – headers already sent by (output started at /home/user1/public_html/wp-config.php:1) in /home/user1/public_html/wp-includes/pluggable.php on line 1196

The error and a blank page appear when i:

1. go to https://www.mydomain.com/wp-admin
2. go to mydomain.com (without www.)
3. go to a raw link without the permalink structure, example: https://www.mydomain.com/?p=21025068

All other sections of the website work just fine – the frontend version with www works (admin does not work), the permalink urls work, for example https://www.mydomain.com/some-permalink

My website is on a Multisite install with subdomains and uses Nginx instead of Apache.

I tried removing all the plugins. I also opened wp-config.php file from multiple text editors to make sure there are no any fault lines in the beginning of it. I also re-uploaded original WordPress files.

Nothing helped. Any ideas what else could i do?

Thank you