As far as I know it’s caused by the sethtmlpageheader show-this-page=1, not sure what the problem there is since I haven’t really looked into the mPDF liberary.
A workaround is to remove the htmlpageheader completly so that it’s just plain php/html. Doing this does make the pagefooter broken so that you don’t have a “page x out of x” anymore. For me this isn’t a problem so I’ve just deleted the whole pagefooter and pageheader tags.
