Error headers already sent in class-IXR.php on line 398

Hi,
I have installed this plugin yesterday. Today, I have created new WP Posts today and I found out that this results consistently in these errors in the httpd error_log:

[Fri Nov 09 09:59:58 2012] [error] [client x.x.x.x] PHP Warning: Cannot modify header information – headers already sent by (output started at /var/www/vhosts/xxxxx/httpdocs/wp-content/plugins/pubsubhubbub/publisher.php:76) in /var/www/vhosts/feestdagen-belgie.be/httpdocs/wp-includes/class-IXR.php on line 395
[Fri Nov 09 09:59:58 2012] [error] [client x.x.x.x] PHP Warning: Cannot modify header information – headers already sent by (output started at /var/www/vhosts/xxxxx/httpdocs/wp-content/plugins/pubsubhubbub/publisher.php:76) in /var/www/vhosts/feestdagen-belgie.be/httpdocs/wp-includes/class-IXR.php on line 396
[Fri Nov 09 09:59:58 2012] [error] [client x.x.x.x] PHP Warning: Cannot modify header information – headers already sent by (output started at /var/www/vhosts/xxxxx/httpdocs/wp-content/plugins/pubsubhubbub/publisher.php:76) in /var/www/vhosts/feestdagen-belgie.be/httpdocs/wp-includes/class-IXR.php on line 397
[Fri Nov 09 09:59:58 2012] [error] [client x.x.x.x] PHP Warning: Cannot modify header information – headers already sent by (output started at /var/www/vhosts/xxxxx/httpdocs/wp-content/plugins/pubsubhubbub/publisher.php:76) in /var/www/vhosts/feestdagen-belgie.be/httpdocs/wp-includes/class-IXR.php on line 398

The error occured whilst creating this new WordPress Post on the following website:
http://www.feestdagen-belgie.be/kies-je-gratis-kalenders-2013-download-printen-blog

I don’t know the consequences nor the cause of this but I have deactivated the plugin now.

I hope the information is useful to you.

—
Thanks for your time,
Rolf.

http://wordpress.org/extend/plugins/pubsubhubbub/