Your instance encountered an error and has closed the connection. Try again or contact customer support. UPSTREAM_ERROR [515]

Please check out the following links where your issue is addressed.

https://aws.amazon.com/premiumsupport/knowledge-center/lightsail-resolve-ssh-console-errors/

https://forums.aws.amazon.com/thread.jspa?messageID=965197&tstart=0

Hello, your question is in regards to Amazon Lightsail. When you receive the error "UPSTREAM_ERROR [515]", then this error is most likely relates to the SSH. Specifically, this "UPSTREAM_ERROR [515]" reveals that your Amazon Lightsail instance cannot be reached while using SSH. Some explanations for this include instance boot failures, instance check failures, resource over-utilization on the instance, an OS-firewall blocking SSH port access, the default SSH port (22) is altered to a different port, or the SSH server is down. To resolve issues regarding the two instance failures or the resource over-utilization on the instance issue, then the status check metrics on your instance should be viewed to determine if the instance failed the system status checks or the instance status check. If the SSH service is down, then restart the SSH. Lastly, if an OS-firewall is blocking the SSH access, then remove the deny rules from the firewall by accessing the instance using terminal or PuTTY or by deactivating the firewall using a launch script if you cannot connect to terminal or PuTTY.

More details can be found here: https://repost.aws/knowledge-center/lightsail-resolve-ssh-console-errors