Wordpress Warning: Cannot modify header information - headers already sent by -

i have such problem in wordpress. when try publish post says warning: cannot modify header information - headers sent (output started @ /home/xxx/domains/colibrilab.net/public_html/mysite/wp-content/plugins/events-manager/classes/em-calendar.php:1) in /home/xxx/domains/colibrilab.net/public_html/mysite/wp-includes/pluggable.php on line 877.

and in line 877 there

header("location: $location", true, $status);

and works on localhost , problem when put live.

what can problem?

function app_output_buffer() {     ob_start(); } // soi_output_buffer add_action('init', 'app_output_buffer'); 

add code in function.php