Symptoms

Either you've recently upgraded or not, if you're on Confluence 3.3 or above:

1. In some cases, clicking the padlock icon or accessing the Restrictions does not yield anything. No pop-up, no error. 
   Firebug or Chrome Developer tools shows 403 on getpagepermissions.action when clicking on the Tools >> Restrictions.
2. The page restrictions dialog box renders blank. The title is 'page.perms.dialog.heading' instead of 'Page Restrictions':
   

Cause

There is a custom Page Layout set either at the space level or global level. Specifically, the custom layout does not include this line of code, which is introduced since Confluence 3.3:

#parse("/decorators/includes/page-content-includes.vm")

Resolution

...
<div id="content" class="page $!mode">
    #parse("/decorators/includes/page-content-includes.vm")

    ## MODE SPECIFIC DECORATING BEGINS
...