It sounds like the problem may be with your browser timing out whilst loading the page (which may be due to a temporary glitch with either your web server or internet connection).
I have just viewed your gallery in Firefox 27.0.1, Chrome 33, Safari 5.1.7, IE11 and Opera 19.0 and it displays and functions OK in all browsers on my PC.
If you continue to experience this issue, try one of the following (although it really should not make a difference as Juicebox should take care of this internally).
(1) Try putting the JavaScript embedding code after the <div id="juicebox-container"></div> line in your web page.
(2) Try enclosing the JavaScript embedding code in a jQuery $(document).ready() function:
$(document).ready(function() {
new juicebox({
backgroundColor: "rgba(51, 51, 51, 1.0)",
configUrl: "config.xml",
containerId: "juicebox-container",
galleryHeight: "100%",
galleryWidth: "100%",
themeUrl: "res/juicebox/jbcore/classic/theme.css"
});
});
Neither of the above should be necessary but if the page is not timing out (which may be the most likely explanation for the problem), then they would certainly be things to try to resolve the Juicebox Error: Cannot find div with id: "juicebox-container".