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UPSTREAM_NOT_FOUND [519] AWS Lightsail SSH not working
Bug Details 
An error occurred and we were unable to connect or stay connected to your instance. If this instance has just started up, try again in a minute or two.

I have faced this issue after upgrading ubuntu from 16.04 to 18.04, then upgraded 18.04 to 20.04.

1) Goto the Lightsail Instance -> Networking Tab -> Switch on the IPV6.

2) Make sure SSH 22/TCP is enabled in both IPV4 and IPV6 without IP restrictions.

3) Restart the Lightsail instance using Reboot or Stop and Start.

4) Click on "Connect using SSH" from the Browser of the Lightsail Instance.
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