New version error

I did the new update today and I’m getting this error when I make a new post:
Warning: call_user_func_array() expects parameter 1 to be a valid callback, function ‘s2_post_updated_send_email’ not found or invalid function name in /home/igneous/public_html/asdf/wp-includes/plugin.php on line 496

Warning: Cannot modify header information – headers already sent by (output started at /home/igneous/public_html/asdf/wp-includes/plugin.php:496) in /home/igneous/public_html/asdf/wp-admin/post.php on line 233

Warning: Cannot modify header information – headers already sent by (output started at /home/igneous/public_html/asfd/wp-includes/plugin.php:496) in /home/igneous/public_html/asf/wp-includes/pluggable.php on line 1178

Its happening on more than one installation of wordpress, and as soon as I disable this plugin the error stops happening

https://wordpress.org/plugins/register-plus-redux/