Shadowbox test results in Page not Found Error

When I click the "Test shadowbox with the current settings." link on the Site Configuration -> Module -> Shadowbox -> Global page, the shadowbox modal dialog is correctly displayed but the content of the dialog is a "Page not found" error. From reading the code, it looks like it should be displaying the sites/all/modules/shadowbox/sample.png file (image of a big Drupal icon).

I have verified that sample.png exists in the directory. Its file permissions are 644.

I read a couple of other issues that I thought might be related: #573320: need detailed instructions on how to use this and #588304: need help to get shadowbox to render correctly in page source. The latter suggested that sites/all/modules/shadowbox/shadowbox.module needed to be modified in the following way. I made this modification but it made no difference so I'm going to revert it to the original:

    // added the missing css file 
    drupal_add_css($module_path .'/shadowbox.css');

    // Initialize Shadowbox.    
    // replaced the Initialize code with the following line
    drupal_add_js('Shadowbox.init(Drupal.settings.shadowbox);',
      'inline','footer');

/* comment out the original init code 
    drupal_add_js('
      Shadowbox.path = Drupal.settings.basePath + "'. $shadowbox_location .'";
      Shadowbox.init(Drupal.settings.shadowbox);
    ', 'inline', 'footer');
*/

I haven't been able to find this particular Shadowbox problem described anywhere and have run out of ideas. If I can't get the test page working I assume I'm dead in the water when it comes to Shadowbox.

Any help you can provide would be much appreciated!
smildrum