Hangouts messages on High Sierra
This is the original fix I saw on the MacRumors forum:

https://forums.macrumors.com/threads/gtalk-messages-solution-found-high-sierra.2073393/


  Go to system settings and go to Internet Accounts and delete your google account.
  
  Open Terminal

find ~/Library/Preferences -name "*iChat*" -delete

  
  then

rm -rf ~/Library/Caches/com.apple.Messages

  
  then

rm -rf ~/Library/Messages

  
  and reboot
  
  Now open Messages and add your google account via Jabber. Then go to preferences in Messages and go to Accounts and go to server settings and make sure settings are as follows:
  
  Server: talk.google.com
  Port: 5223
  Check Use SSL
  
  Then go to Account Information and Enable this account
  
  I rebooted again and then opened Messages and made sure my buddy list was there. 
  
  Then I went to System Preferences and go to Internet Accounts and add your google account.
  
  Upon doing this a message popped up for me asking to Add Google Talk to Messages and WHOA it is working.
  
  I can now send and receive messages, have my calendar and everything as before.
  
  Before this I could receive messages and see buddies but couldn't communicate with them.