Error message when logging in

I had a query earlier which appeared to be resolved. http://ww.wp.xz.cn/support/topic/site-goes-to-home-page-when-i-try-to-save-settings?replies=10

However now my site http://www.landlordlawblog.co.uk (although it seems to be displaying OK for readers) is now showing an error page when I try to log in or (in the browser where I am already logged in) try to save anything.

This is the message:

Warning: Cannot modify header information – headers already sent by (output started at /home/tessashepperson/public_html/wp-config.php:83) in /home/tessashepperson/public_html/wp-includes/pluggable.php on line 876

I have had a look at that file and that section – linie 876 says

header(“Location: $location”, true, $status);

And is part of the following:

if ( !function_exists('wp_redirect') ) :
/**
 * Redirects to another page.
 *
 * @since 1.5.1
 * @uses apply_filters() Calls 'wp_redirect' hook on $location and $status.
 *
 * @param string $location The path to redirect to
 * @param int $status Status code to use
 * @return bool False if $location is not set
 */
function wp_redirect($location, $status = 302) {
        global $is_IIS;

        $location = apply_filters('wp_redirect', $location, $status);
        $status = apply_filters('wp_redirect_status', $status, $location);

        if ( !$location ) // allows the wp_redirect filter to cancel a redirect
                return false;

        $location = wp_sanitize_redirect($location);

        if ( !$is_IIS && php_sapi_name() != 'cgi-fcgi' )
                status_header($status); // This causes problems on IIS and some FastCGI setups

        header("Location: $location", true, $status);
}
endif;

I am not a programmer so am not able to fix this. Do you have any ideas?

Shortly before the crash I had installed a replacement plugin for wordpress firewall 2 which was Better WP Security. Maybe that had something to do with it. I did not do the settings however and was trying to remove it when the problems started.