SOLUTION
After running a comparison with a working version (on Sierra 10.12.6) it looks like the solution (at least for me) is to change the Where value that gets generated on sign in from what gets populated when adding an account in High Sierra com.apple.account.jabber.password to Jabber
In order to make this change follow these steps:
- Open Keychain Access
- Search for com.apple.account.jabber.password
- Double click it to open the attributes prompt
- In the Where field (fourth one down) you should see com.apple.account.jabber.password
- Replace all text in that field with Jabber
- Click Save Changes
After making this change the first launch of messages asked me to allow permission to access keychain. After doing so any subsequent open of messages works as expected.
I hope this helps.