Hi,
Usually this error occurs when the web driver fails to connect to the browser or gets disconnected from the browser, what ever the cause it's outside robot framework, you will look into what is happening with chrome on your test machine.
I have seen this error when there is too applications consuming resources on the test machine and the browser crashes or fails to launch due to lack of available memory or when the browser is running too slow due to high cpu usage, but there is no way from robot framework to know what the cause was.
Hope that helps,
Dave.