I stopped/started the instances and I was able to connect. Don't know if they resumed the prior reboot command or what, so I attempted to reboot again and they completed this time. A big mystery. Doesn't instill confidence.
So, the solution Amazon provides us is the Microsoft mantra, reboot/restart/reboot/restart/re<verb>/re<verb>/re<verb>... until the thing starts working again. The future is here, people, and it requires frequent starts!
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