症状

1. You are logged in as a Confluence Administrator
2. You cannot navigate to Confluence Admin as it's missing the Administration Cog:
   
3. Navigating manually to <yourConfluenceURL>/admin however, works, and brings you to the Confluence Administration console
4. There are no customizations such as CSS, Custom HTML, or plugins that might have hid the element

原因

The Administration Menu Items plugin in Confluence Admin >> Manage Add-Ons >> System is disabled

ソリューション

• Navigate to the Confluence Administration console manually via <ConfluenceURL>/admin
• Then navigate to Manage Add-Ons , and select System from the drop down menu (the default selection is Action Required)
• Look for the Administration Menu Items plugin, click on it, then Enable the plugin and all plugin modules