When I view your website on my iOS device, the gallery's Splash Page is displayed as expected (please see the Screen Modes support section for more details on the Splash Page) but when I click the Splash Page to expand the gallery, I see the following error message:

When a gallery is expanded on an iOS device, the gallery is expanded on a new page instead of on top of the embedding page (please see the Expand Gallery Behavior support section) and Juicebox uses the 'full.html' file inside the gallery's 'jbcore' folder to display the gallery.

The problem appears to be that the 'full.html' file does not seem to exist in its correct location on your web server.

Your galleries share a 'jbcore' folder in your root directory so the 'full.html' file should be located here: http://www.arthouse.org/jbcore/full.html
However, if you go directly to this location in a browser, you'll see the same error 404 message as when trying to expand the gallery from the Splash Page, suggesting that the file is not there.
Normally, when you open a gallery's 'full.html' file directly in a browser, you'll see the "Juicebox Error: Config file not found." message (the file should be used internally by Juicebox only) but you should not see an error 404 (file not found).

Try reuploading your 'jbcore' folder to your web server and make sure that it contains the 'full.html' file.
If you are sure that the file is in the correct location, then check its permissions to make sure that they are not too restrictive. (Default permissions of 644 should be fine.)
If you are sure that the file is in the correct location and that its permissions are OK, then I would ask your web host why the 'full.html' file returns an error 404 when going to http://www.arthouse.org/jbcore/full.html

Hopefully this will point you in the right direction.