Error headers: ap_headers_output_filter() after putting cache header in htaccess file
Generally since this is a debug level error/notice, you can turn it off in Apache config (perhaps /etc/apache2/sites-enabled/somesite, /etc/apache2/httpd.conf or /etc/apache2/apache2.conf or perhaps a .htaccess file) by setting a line to:
LogLevel warn
as opposed to:
LogLevel debug