This usually means one of 2 things.

1/ You have a corrupted file - download a fresh copy of your version from the downloads page and upload the catagories plugin file to your server again.

2/ You've been playing with the file - Either undo the changes you've made or post the code from around that point.

¥

I guess they don't call you Guru for no reason! *does the happy dance* You're brilliant, I did as you suggested and all is good with the world, thank-you! ;)

i have the same problem, i downloaded the new file from your server and uploaded the file again, no changes.

i get the error message, when i try to create the static file.

Warning: Cannot modify header information - headers already sent in /var/www/virtual/dejung.net/may25th_de/htdocs/inc/VIEW/_menutop.php on line 38

if i click on the new created file, is a other warning message on the top of the page......

Warning: Cannot modify header information - headers already sent in /var/www/virtual/dejung.net/may25th_de/htdocs/skins/kubrick2evo/_main.php on line 34

the important part of the _main.php looks like:

// aligns your bloglist to the left or centered or to the right
$BlogList_Align = 'C'; // can be L or C or R
// ****************************************************************************

header( 'Content-type: text/html; charset='.$io_charset );
?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="<?php locale_lang() ?>" lang="<?php locale_lang() ?>">
<head>

the important part of the _menutop.php looks like:

 * @author mbruneau: Marc BRUNEAU / PROGIDISTRI
 *
 * @todo Let the {@link AdminUI_general AdminUI} object handle this. NEEDS MASSIVE CLEANUP!!!!
 */
if( !defined('EVO_MAIN_INIT') ) die( 'Please, do not access this page directly.' );

global $io_charset, $rsc_url, $UserSettings, $Debuglog, $Plugins;

header( 'Content-type: text/html; charset='.$io_charset );
?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">

have anyone an idea?

It does not help to quote the place, where the warning happens (because the real error, which is "headers already got sent") is before.

I've created a FAQ entry for it, because it happens so often:
http://manual.b2evolution.net/FAQ#.22Cannot_modify_header_information.22

Unfortunatly you don't have the "(output started at /path/to/file/with/output:LINE)" part in your warning..
But still, the main message remains: you've probably edited a file, e.g. added newlines at the end, which triggers output to be sent to the browser, before it comes to the header() calls you've quoted above.