The error message has nothing to do with this.
The frontend connects with a websocket to the server. Everytime something disconnects it, you get the connection lost message and after a few seconds it will reconnect and work goes on. I hope this also happens for you.
If it starts coming and going it might be a instable wifi connection of the pi. You could then test with cable connection to verify it is the reason.
If not provide more informations like you os and browser type/version you are using. We develop it with chrome and have the least problems with that combination, also firefox and new IE also work.
