Cannot Access WordPress admin

Cannot access WordPress admin dashboard, tried disabling the themes, and plugin folder via FTP, and still cannot access the dashboard. .htaccess file looks good.

Debug logs generates the following errors:
[13-Aug-2017 03:04:42 UTC] PHP Warning: Cannot modify header information - headers already sent by (output started at /home/vellumvo/public_html/wp-config.php:1) in /home/vellumvo/public_html/wp-includes/pluggable.php on line 1210