I received this error yesterday when a new team added my bot and my app tried to start the bot for the first time:
level=error msg="starting client" error="unexpected end of JSON input" team=TXXXXYYYY

The troubling thing was that relax didn't automatically try to restart the bot the way it does for other errors. So I tried restarting the bot manually (with Relax::Bot.start!(uid, token)) and I got the same error. So then I restarted Relax which seemed to set things right.