Steps to reproduce the problem:

1. Create a HC user using HC's REST API (this ensures an Atlassian account is not created).
2. Approve the HC user's email manually (I used the "ops" admin console for this)
3. Log in to HipChat in www.hipchat.vom/sign_in. It will redirect you to the "We're joining your accounts..." page.
4. Enter your password there and you'll be redirected back to HC login page without getting logged in