aws lightsail - cannot Connect using our browser-based SSH client.

It sounds like you're experiencing connection issues with your Amazon Lightsail instances. This can be frustrating, but there are a few things we can consider to troubleshoot the problem:

1. Connection errors with browser-based SSH client: When you can't connect using the browser-based SSH client, it could be due to a few reasons. One possibility is that the instance hasn't fully started after being created, rebooted, or restarted. In this case, you might see a "Can't connect right now" error message. If this happens, wait a few minutes and try to reconnect.

2. Host key or certificate mismatch: Sometimes, connection issues occur due to a host key or certificate mismatch. This can happen after an operating system upgrade or if the host key was deliberately updated. In such cases, you might need to reset the known host key or certificate using the Lightsail console or AWS CLI.

3. Firewall issues: Connection timeouts or refused connections can be caused by firewall settings. Check if Lightsail's firewall or the operating system-level firewall is blocking access on the necessary ports (typically 22 for SSH).

4. Application or operating system issues: Ensure that the SSH service is running on your instances. There might be an operating system-level issue preventing the service from starting properly.

5. Network restrictions: Although less likely in your case (since one new instance is working), it's worth checking if there are any local network restrictions blocking your access.

Since you've already tried rebooting, stopping/starting, and even creating new instances from snapshots without success, and given that a newly created instance is working, it suggests there might be an underlying issue with the affected instances themselves.

Here are some additional steps you can try:

1. Check the instance logs and metrics in the Lightsail console for any unusual activity or errors.
2. If possible, try connecting from a different network or device to rule out local network issues.
3. Verify that the instances' security group settings allow inbound SSH traffic.
4. As a last resort, you may need to contact AWS Support for further assistance, especially if you suspect there might be a platform-level issue affecting multiple instances.

Remember, even if you can't connect via SSH, you should still be able to manage your instances through the Lightsail console, including taking snapshots to preserve your data.

Sources
Resolve connection errors with Lightsail browser-based SSH and RDP clients - Amazon Lightsail
Connect to Linux or Unix instances on Lightsail - Amazon Lightsail
Troubleshoot connection errors | AWS re:Post
Instances - Amazon Lightsail

If you are unable to establish an SSH connection, first check if your instances are exceeding their burst capacity: https://docs.aws.amazon.com/en_us/lightsail/latest/userguide/viewing-instance-burst-capacity.html https://docs.aws.amazon.com/en_us/lightsail/latest/userguide/troubleshooting-high-cpu-utilization.html

If your instances aren't running into burst capacity issues, confirm that port 22 is open on the firewall: https://docs.aws.amazon.com/lightsail/latest/userguide/amazon-lightsail-editing-firewall-rules.html

If neither of these suggestions help you resolve your issue, please provide more details about what happens when you try to connect.