The message "This instance is not registered with Google Drive Plugin" is showing for embedded Google Docs

Summary

The message "This instance is not registered with Google Drive Plugin."is showing for embedded Google Docs.

Also, the following error is shown while trying to install "Confluence Google Drive":
UI:
The add-on host returned HTTP response code 500 when we tried to contact it during installation. Please try again later or contact the add-on vendor.

Logs:
2016-02-09 14:16:43.834229500 2016-02-09 14:16:43,833 ERROR [UpmAsynchronousTaskManager:thread-4] [connect.plugin.lifecycle.ConnectAddonManager] callSyncHandler Error contacting remote application at https://confluencegoogledrive.uswest.atlassian.io/installed?user_key=ff808081503e546001503e547d410002 500:[Internal Server Error

Steps to Reproduce

• Open a page that has a Google Doc embedded
  OR
• Try installing "Confluence Google Drive"

Expected Results

Add-on is installed and embedded files are loaded

Actual Results

This instance is not registered with Google Drive Plugin

Workarounds

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