Cannot send session cache limiter - headers already sent

Hi, how you doing?

A costumer ask me to help him to fix a problem with WP 4.1.1.
When click to edit a post or page the server return a white page.
For example

/wp-admin/post.php?post=1602&action=edit

So I enabled the "debug mode" for a while in order to see what's going on.

The message error is:

<b>Warning</b>: session_start() [<a href='function.session-start'>function.session-start</a>]: Cannot send session cache limiter - headers already sent (output started at /..../public_html/wp-includes/functions.php:3547) in <b>/......./public_html/wp-content/plugins/custom-sidebars/inc/class-custom-sidebars-explain.php</b> on line <b>55</b><br />

The plugin is "Custom Sidebars" version 2.1.0.2.

Thanks for your help.
Regards