After locking down the system according to your configuration, we are able to reproduce the issue you are referring to. Also, the issue doesn't seem to be related to hiding the navigation buttons or address bar as the hang happens when those features are revealed. It seems more likely that the webpage is searching for something it does not have access to while the browser is locked down. Also note that the widget that seems to be locking on this page does not lock the browser when displayed on other pages.
Without having access to the website code, it is unclear what is causing this hang to occur. You may want ask the developer, or try using a different browser like Chrome which seems to work fine in our testing. In Chrome, you cannot remove the address bar, but you can restrict users from going anywhere beyond the pages you specify in the Secure Lockdown > Chrome Lockdown > Allowed Sites section.