Hi,
for some (yet unknown) reason loading the images from dl.dropbox.com fails. When the images are served from somewhere else the header shows up correctly:
$client->setHeaderHtml('<div id="header" style="background-image: url(http://s3.pdfcrowd.com/public/misc/header.svg); height: 105px; overflow:hidden;">
<div id="globe" style="float: right;">
<img src="http://s3.pdfcrowd.com/public/misc/globe.svg" />
</div>
</div>');
We will investigate this further but for now it seems the workaround is to move the images referred from header/footer code from dropbox to somewhere else.