It seems like I cannot send messages on Google Hangouts via Messages after upgrading to High Sierra.
I still receive the messages because I'm authenticated.
Is there any hope to keep writing in Hangouts via Messages?
This is the original fix I saw on the MacRumors forum:
Go to system settings and go to Internet Accounts and delete your google account.
find ~/Library/Preferences -name "*iChat*" -delete
rm -rf ~/Library/Caches/com.apple.Messages
rm -rf ~/Library/Messages
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:
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.
I actually have no idea about messages (I'm sorry about it). But the third party app Franz may help you to get out 'till someone may can help you.