For the past week or so (I don't use the option too often so I don't have a good idea when it stopped working), whenever I click on the "Customize" link under the Appearance category the customize.php page loads with the sidebar on the left, but the links aren't clickable and the page preview doesn't load on the right side. I tried copying a version of customize.php from a backup from about a month ago (file itself dated in November 2013), but no changes. I've tried loading the page in Chrome and Firefox with the same issue. Any ideas what could be the cause? Thanks.