Thank you very much for sending the log file.
We are currently analyzing it. It seems that the server is disconnecting due to network connection lost.
Can you quickly try with spacedesk HTML5 Viewer?
Just open a Google Chrome web browser on your Chromebook device then go to http://viewer.spacedesk.net/ and try to connect to your server IP address. Then let us know if the display off / black screen is also happening.
Please also make sure that you update your NVIDIA GeForce GTX 1660 Ti graphics adapter to the most recent version.
You can download the latest driver installer from their website https://www.nvidia.com/download/index.aspx.