I have heard back from AWS support on this. They report:
"I'd like to inform you that the team has reverted with an update. The team has stated that they have made some troubleshooting steps and they have resolved the issue."
And sure enough, it is now working as expected. So.... Attention future folks! Make sure that you've taken all the troubleshooting steps necessary, like NACL review, public IP for instance, Security Group open enough, internet gateway on the subnet, and any other possible technical reasons for failure. Then, even with the free 'basic' account, enter an "account and billing" ticket, explaining the problem. Then a few days later, after some back and forth, they'll fix your account.
I wonder if they isolated my account by accident?
Seven minutes is an odd time; especially if it is consistent across operating systems; and that the instances continue to communicate even thought you can't connect to them.
Are you using a restrictive inbound security group for the instances? Is it possible that your public IP is changing (your ISP; upstream NAT devices) and that's causing the issue?
Thanks for the response. I've got no restrictive inbound security group and the NACL is wide open, too. Security group is set to allow inbound port 22 from 0.0.0.0/0.
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The strangeness increases. Now I'm not able to connect to any instances at all. I've verified a public IP on a new instance, the Subnet has 0.0.0.0/0 routing to an internet gateway. Security group is wide open to ssh (0.0.0.0/0 in bound port 22), I've been working in us-east-1, but tried us-west-1, as well.
I've setup a new VPC, subnets, and IGW. Same results. I can connect to other accounts instances.
What instance type(s) are you using?