It looks like there is something not right with your wp-includes/formatting.php file.
The “Headers already sent” error is usually because there are spaces, new lines, or other stuff before an opening <?php tag or after a closing ?> tag. Have you made any changes to this file? One easy way to make sure everything is back to original is by replacing that file with one from a fresh download of WordPress.
More on the Headers already sent error:
https://codex.ww.wp.xz.cn/FAQ_Troubleshooting#How_do_I_solve_the_Headers_already_sent_warning_problem.3F
Dear Geoffrey,
I really appreciate your help. Thank you!
This is my formatting.php. I cannot see anything broken there. Do you?
https://gist.github.com/orschiro/2fa17469835f936d7ea6a5074170b680
Sorry for the delay in getting back with you! As long as you don’t have any extra new lines at the beginning or end of the file, it looks fine to me.
I would try manually re-installing WordPress and see if you are still getting those messages.
If that doesn’t work either, you could try deactivating all plugins. If that resolves the issue, reactivate each one individually until you find the cause.
If that still does not resolve the issue, try switching to the default theme for your version of WordPress to rule-out a theme-specific issue (theme functions can interfere like plugins).
Dear Geoffrey,
Thank you!
It is a plugin problem and I am right now isolating which one it is. 🙂
Yours,
Robert
