If you’re a Google Apps user, you may want to hold off on upgrading to the newest version of Hangouts (version 4.0) on Android; a strange glitch means some users can’t connect to Google’s chat server.
According to Android Police, if your Google Apps account has had Hangouts disabled by an administrator, you won’t be able to use Hangouts for any account on your phone.
In other words, if you use your phone to check your work email, but an admin has disabled Hangouts for that account, you won’t be able to chat using your personal email either, or any other Google emails on your phone.
Image credit: Android Police
Obviously, that shouldn’t be the case – and indeed if you revert back to Hangouts, 3.3 the problem goes away.
Your only other solutions appear to be completely removing the offending work account from your device, or enabling Hangouts via the Admin console if you happen to have access for that account.