Let me jump in on this one since I had the same problem and found a (rather simple) solution for it.
At the very top of my dump.xml file, for some reason, there was the following snippet
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
<?
$wgExtensionCredits['wikidiff2'][] = array(
'name' => 'Wikidiff2',
'version' => '',
'author' => 'Tim Starling',
'description' => 'external diff engine for MediaWiki',
'url' => 'https://www.mediawiki.org/wiki/Extension:Wikidiff2'
);
$wgExternalDiffEngine = 'wikidiff2';
and only after came the parent xml tag:
<Mediawiki xmlns="http://www.mediawiki.org/xml/export-0.6/" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://www.mediawiki.org/xml/export-0.6/ http://www.medi\
awiki.org/xml/export-0.6.xsd" version="0.6" xml:lang="en">
So what i did was just remove everything the was before that parent tag, and it imported without a problem.
hope it helps.
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