What does the ERR_NOT_IMPLEMENTED error mean when connecting a WS client to the server?

Question

JavaScript

I am writing a websocket server and when connecting to the websocket server the following error occurs:

websocket connection to ... failed: Error in connection establishment: net::ERR_NOT_IMPLEMENTED

This error is reproduced only by one of the users of the service that I am developing. Google chrome browser version 44.0.2403.157m
Judging by the logs, he is far from the only one who uses this version of chrome, but there were no more complaints from anyone.
What could this error message mean.