In the instance of a hard crash like this, the best thing to do is to hook up a monitor and keyboard to the system and boot into non-GUI mode so you can tail the log while the server is running.  This will print every log event directly to the monitor and when it crashes, you can take a picture of the last messages in the log so we can see what is causing the crash.  See this FAQ in the wiki for instructions:   https://wiki.unraid.net/Unraid_6/Frequently_Asked_Questions#My_system_is_cr