Unable to login – Warning: Cannot modify header information

It looks like this site has been hacked. WordPress version is the second most recent – issues discovered when I went to backup and upgrade to the newest version.

The site loads and then shows McAfee warnings when any internal link is clicked. It is not possible to log in. Attempts to login yield repeated “Cannot modify header information” errors and the WordPress error of “Cookies are blocked due to unexpected output. ”

Disabling all plugins from the control panel doesn’t make any difference. As far as I can tell only the normal files and directories exist. I have full backups of the site saved with Updraft. Any advice would be appreciated.

The full error message is:

Warning: Cannot modify header information – headers already sent by (output started at /home/username/public_html/wp-config.php:2) in /home/username/public_html/wp-login.php on line 402

Warning: Cannot modify header information – headers already sent by (output started at /home/username/public_html/wp-config.php:2) in /home/username/public_html/wp-login.php on line 415

Warning: Cannot modify header information – headers already sent by (output started at /home/username/public_html/wp-config.php:2) in /home/username/public_html/wp-includes/pluggable.php on line 892

Warning: Cannot modify header information – headers already sent by (output started at /home/username/public_html/wp-config.php:2) in /home/username/public_html/wp-includes/pluggable.php on line 893

Warning: Cannot modify header information – headers already sent by (output started at /home/username/public_html/wp-config.php:2) in /home/username/public_html/wp-includes/pluggable.php on line 894