Thank you so much for your reply!  
Beyond the ZMQ connection I had a very fuzzy idea of what was going on. 
Your explanation has started to fill in a lot of gaps.

I fixed the problem last night - it turned out to be a threading issue.
When I rapidly evaluated notebook cells (hitting shift-enter really fast) 
the Python in Jupyter Notebook would crash as shown.
Multiple processes were calling the pzmq:send function at the same time and 
the messages were becoming interleaved and garbled.
I wrapped a mutex around the function that calls pzmq:send multiple times 
and the problem went away.

On Monday, December 18, 2017 at 2:58:08 AM UTC-5, Roland Weber wrote:
>
> Hello Christian,
>
> the architecture looks like this:
>
> Browser <-> Jupyter Notebook <-> Kernel
>
> The browser communicates with the Jupyter Notebook server using HTTP and 
> Websockets.
> The notebook server communicates with your kernel using ZMQ connections.
> In other words, the notebook server translates between websocket 
> connections and ZMQ connections.
> It's written in python, and that's where the Jupyter Client is being used.
>
> With Jupyter Kernel Gateway, which plays a similar role as Notebook for 
> non-browser clients, we've encountered invalid signatures when a kernel 
> used the wrong character encoding. Are you using utf-8 consistently when 
> processing messages?
> https://github.com/jupyter/kernel_gateway/issues/241
>
> hope this helps,
>   Roland
>

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