Steps to reproduce

1. Go to Dashboard > Administration > Plugin Manager, click on Confluence Usage Stats plugin
2. If it's enabled, disable it
3. Click on Enable plugin
4. Refresh the page, it should bring you to something like this:
   4.Refresh_After_Enable.png
   If you double-clicked on Enable plugin in Step 3, you will get the same thing too.
   And also notice that under the plugin name at the left side, it's written Errors loading plugin like this:
   Errors_loading_plugin.png
   1. Click on Enable plugin, you will get an error message like the following:
      4.1Enable_After_Refresh.png
   2. Or refresh the page again, you will get the same error message
5. After getting the error as described in step 4.1 or 4.2, an Uninstall plugin option appears. AFAIK, bundled plugins should not have an Uninstall plugin option. If you click on Uninstall plugin, you will get a System Error page:
   5.System_Error.png

Workaround

• A restart of Confluence instance will make the plugin functional again.

Findings