cannot modify header warning??

What is this?

Warning: Cannot modify header information - headers already sent by (output started at /home/trac7032/public_html/leadingfromtheheart.org/wp-content/themes/scrapbook/functions.php:36) in /home/trac7032/public_html/leadingfromtheheart.org/wp-includes/pluggable.php on line 865

All of a sudden this morning I get this error each time I make any kind of modification to my blog (add a new post, activate/deactivate plugins).

For example, I made a new post. Through the writing, as it refreshed, this warning would appear in pink below the content input box. Then I clicked publish and was brought to a page that had only the warning at the top of it, nothing else. But then if I retype in my blog address, the post is posted.

When I try to deactivate a plugin, I am brought to the warning page. If I renavigate to the plugin page, I see that the plugin is indeed deactivated.

Is this a 2.8.4 thing?

It coincided with an All in one SEO plugin update. I deactivated it, but it is still happening.