I wasn't able to reproduce this issue with the Linux and Windows versions either, but I remember vaguely that something like that happened to me once when I pasted metadata entries outside the header section and/or deleted a required tag.
@tetrault: BTW, most of your header information will be ignored by epub apps. You only need a title tag, which can be empty, because it won't be displayed by epub apps anyway and link tags for stylesheets etc.:
Code:
<head>
<title></title>
<link href="../Styles/styles.css" rel="stylesheet" type="text/css"/>
</head>
(Without an empty title tag, your book will fail epubcheck.)