If you are a Google Apps for Businesses customer you might see a red error messaging stating “Unable to sign in :(” in the Hangouts sidebar, detailing:
“Hangouts has not been enabled for your account. Please contact your administrator to enable this service.”
To get rid of it, in the classical admin panel to to Settings > Talk and activate the feature called “Enable the new Hangouts for messaging and video calls”.
The Google+ help might wrongly point you to the Services > Google+ section. I hope this helps others stopping the frustration. If you want to stay in touch, I’m on Google+.