messages fails instantly on high sierra

I installed High Sierra last night. Since then trying to send messages through iMessage on my MacBook Air fails instantly. The little red "not delivered" icon displays instantly upon sending. However, new messages come in just fine and sending messages from my iPhone show up as sent just fine on MacBook Air. I simply can not send new messages from the MacBook Air.

Tried so far:

restarting messages, logging out and back into iCloud on MacBook, turning off iMessage and back on again, rebooting. What did I miss?