Output breaks after installing bad-behaviour plugin

Hi

To reduce incoming spam comments I installed Bad Behaviour. As I was logged in I did not notice it until the next day – anonymous request (not logged in) result with an empty page. The output ends right before the “</head>” element AFAI understand.

I tried to search the code for errors… and found out that in the header.php file of the template that is used there is a call to wp_head that terminates the output. There is no output after the wp_head function call.

<link rel="pingback" href="<?php bloginfo('pingback_url'); ?>" />
<?php wp_head(); ?>
</head>

At the same time, for a logged in user everything is displayed correctly.
I have disabled BadBehaviour and renamed the folder (to disable all the scripts that might have caused the problem) … nothing.

What could be the problem?

Thanks.