Late reply, again. Stumbled upon this issue when trying to connect to WSS websocket in another subject. Turned out that I've been trying to use the wrong certificate files, at least this time.
Note to self (and others with same issue); Use the correct certs, and make the haproxy.pem like this if you're using Let's Encrypt / Certbot:
cat fullchain.pem privkey.pem | tee haproxy.pem
This time I got it working with the config from Kazoo docs, don't know if your settings will work also, @RuhNet (it probably will);
https://docs.2600hz.com/supported/applications/blackhole/doc/#wss-considerations