Finally solved this one, I was mistakenly sending the websocket’s state instead of the json serialized payload. Now it’s fine, thanks again @al2o3cr. However, this error stack so cryptic and unfriendly for (just) Elixir coder. 
So, you’re right, I need to get my hands dirty with Erlang. In this case I need to understand the underlying implementation of this bloody cowboy_websocket. 
…iodata and such.
I consider a huge drawback for Elixir that is based so much on Erlang’s libraries. (immaturity ?). This remembers me Java in the late '90s. Or C#'s infancy.
