Since upgrading HTTPS to 3.1, a small incompatibility is exhibited between this plugin and the Atahualpa theme. The HTML for the header image somehow loses a single quotation marker, so no image appears. As a specific example, when the HTTPS plugin is enabled, the following HTML is output on the front page of faculty.missouri.edu/~umcasphysics:
<div id="imagecontainer" class="header-image-container" style="background: url('http://faculty.missouri.edu/~umcasphysics/wp-content/themes/atahualpa/images/header/banner2.gif) top center no-repeat;">
Notice in the url( ... ) styling, the single quote is missing after banner2.gif.