Google switched to a newer format to Google Plus a couple of days ago. This made our users see a red box that says “Unable to sign In :( Hangouts has not been enabled for your account. Please contact your administrator to enable this service.” and access to hangouts was effectively turned off for all users in our domain.
I have now enabled the new Hangouts for our domain.
Here are some of the features of the new Hangouts that Google lists:
Receive notifications across devices
Users will receive a notification across all of their devices when a message has been sent.