Have a look at this post. It helped me solve the same problem I had with the header issue.
The key thing to check is the file in the first part of the error (I bolded it):
Warning: Cannot modify header information – headers already sent by (output started at /home/optimalw/public_html/wp-settings.php:473) in /home/optimalw/public_html/wp-includes/pluggable.php on line 850
If you open your wp-includes.php file in a plain text editor, you’ll probably notice extra whitespaces and line breaks. Remove those whitespaces, save your file, and upload it.
It will hopefully solve your problem for that error.
