This URL looked good to me, so I am not entirely sure what the problem would be at this stage unfortunately 
If the n8n desktop application hasn’t simply crashed, I wonder if any firewall, proxy, etc. in your network might have managed to block incoming requests here. Do other requests sent to your n8n desktop instance work for you?
E.g. if you create a workflow with only a webhook node and execute the workflow, would you be able to open the webhook URL in your browser (or would it return the same 404 you are getting when authenticating with Typeform)?