I believe I've been seeing the same thing as the OP. I don't read in calibre, and I don't get the error anywhere else, so I've pretty much just been ignoring it so far.
But I can tell you it started with either version 0.9.7 or 0.9.8. Books that had shown no errors before that started showing the error when viewed in those versions and later--and it's a LOT of books, all from retail copies. It's always on the cover page. Epubcheck it says they validate, fwiw.
Here's an example. I don't really know what to look for as far as problems.
<html xmlns="http://www.w3.org/1999/xhtml" xmlns:svg="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink" xml:lang="en-gb">
<title>A Book Title</title>
<link href="../styles/0000000000000.css" rel="stylesheet" type="text/css"/>
<meta content="urn:uuid:00000000-0000-0000-0000-000000000000" name="Adept.expected.resource"/>
<svg:svg viewBox="0 0 1243 1600">
<svg:image height="1600" transform="translate(0 0)" width="1243" xlink:href="../images/0000000000000.jpg"/>