Google Hangouts integration cannot be installed

Summary

When attempting to install the Google Hangouts integration at https://example.hipchat.com/addons/, the Add Integration button is grayed out. The problem is present when attempting to install the integration Globally (All rooms) or for a particular room.

Environment

• HipChat.com

Steps to Reproduce

1. Log into HipChat.com as a Group Admin or Owner user
2. Navigate to Integrations
3. Scroll down to choose the "Google Hangouts" integration

Expected Results

Add Integration is clickable and installation proceeds.

Actual Results

The Add Integration button is grayed out and cannot be selected. Hovering over the link provides the following output:

Problem retrieving capabilities descriptor at https://marketplace.atlassian.com/downloads/plugins/com.atlassian.hipchat.hangouts/version/100: Unexpected response 400

Workaround

1. Log into HipChat.com as an Admin or Owner user
2. Navigate to Integrations
3. Scroll to the very bottom of the screen and click Install an integration from a descriptor URL
4. Enter https://hangouts.hipch.at/ and click Install