From the error message is seems that many connection attempts fail through a websocket. This might be for different reasons related to connectivity or the VM. As this happens after several days of training, the most probable theory is "VM related "such as if the disk is out of space, utilization, or an issue with the proxy. Here is a
guide for troubleshooting that should get you started. To explain, " Inverting Proxy agent runs inside your Notebooks instance as a Docker container" and is responsible for the connectivity. The factors above can affect it and cause such issues. I hope this puts you on the right track. I suggest posting on
serverfault.com if you find more specific logs if you SSH to the instance (from the guide above) and to help the community troubleshoot.