There is extra whitespace at the beginning or end of one or more of your PHP
files, probably LocalSettings.php or another file you have edited. Remove it.

webmaster wrote:

This happens at a fresh and clean Installation with the Installer.

Your wiki has an extension in it, so it's clearly not a fresh and clean
installation. Look at the file.

webmaster wrote:

It only the SitemapExtension :

<?php

$wgExtensionFunctions[] = "wfExtensionSpecialGoogleSitemap";
$wgExtensionCredits['specialpage'][] = array
(

'description' => 'Adds a special page to create a XML Google Sitemap

file, along with some reporting.',

'url' => 'http://www.mediawiki.org/wiki/Google_Sitemap_Extension',
'author' => 'François Boutines-Vignard',
'version' => '0.0.4'

);

function wfExtensionSpecialGoogleSitemap()
{

global $wgMessageCache;
$wgMessageCache->addMessages(array('googlesitemap' => 'Google Sitemap')); 

$wgAvailableRights[] = 'googlesitemap';
$wgGroupPermissions['bureaucrat']['googlesitemap'] = true;

SpecialPage::addPage( new SpecialPage( 'GoogleSitemap' , 'userrights') );

}

?>

robchur wrote:

As above, please check for whitespace or other characters, such as byte-order
marks, at the start of the file(s) you have introduced or changed, and at the
end, after the ?> tag.

webmaster wrote:

I did not have anything done like this !!!

robchur wrote:

You may not have intentionally done anything; you may have accidentally
introduced whitespace or other noise during the upload, if you uploaded the file
to a remote server. If you edited the file in an application which inserts
byte-order marks in files, this may have introduced something into
LocalSettings.php. This, however, is where the problem is coming from, so you
need to check any files you have modified, both core and extension.