Warning: Cannot modify header information – headers already sent by

check this thread

https://www.ads-software.com/support/topic/warning-cannot-modify-header-information-headers-already-sent-by-2?replies=27

disable all plugin and enable one by one , may be its plugin issue

or try to change your theme

I believe you might have a bad install. Please try re-uploading WordPress Core files to your server from a freshly downloaded copy off www.ads-software.com. You will need to agree to replace duplicate files with the uploaded versions (note that if you’ve changed any files which are supplied by the distribution package then these will get reverted to their original state). Doing this should ensure that wp-includes/sesstion.php is not mal-formed in some manner.

Place the function ob_start(); top of the functions and end with
return ob_get_clean();

Thanks Daniel Llewellyn
I had to re-upload WordPress Core files taking a backup of wp-content and database first.

My site seems to be hacked again with the same message as before:

Warning: Cannot modify header information – headers already sent by (output started at /home/carryonweb/public_html/wp-includes/class-walker-comment.php:1) in /home/carryonweb/public_html/wp-includes/pluggable.php on line 1171

Is there something wrong with WordPress security. I updated to the newest version.

lynnjenkins, I got the exact same problem. Have you solved it? If so I would appreciate your help more then anything right now. This is killing me…

This is likely the result of a plugin or your theme. Disable all plugins and revert to a twenty* theme to see if that fixes the problem. Then re-enable each one-at-a-time until you recreate the issue. You should then know which add-on caused the problem and can start to diagnose where the rogue output came from.

alaxandercibic, I re-installed wordpress after taking a backup of wp-content and the database first. I used filezilla to delete and upload files. Hope this helps. Good luck with everything.

Thank you for answering. Is this possible without filezilla?

You may find this article useful – https://codex.www.ads-software.com/WordPress_Installation_Techniques . If you scroll down you will see:
‘Installing WordPress Locally’ and ‘Using Various Web Hosting Interfaces’