After spending a night to figure out how to get the (video) chat running in a Windows Server environment and SSL I just wanna share my result.

• install websockets
• install ARR Cache
• install URL rewrite
• install a SSL cert via Let's Encrypt and bind it properly

ARR config: enable proxy
under Proxy type:
check "use URL rewrite to inspect..."
check Enable SSL offloading
reverse proxy: 127.0.0.1:8069

URL Rewrite
Rule #1: ReverseProxyWebSocket

regular expression: ^(.*)$
condition: {HTTP_UPGRADE} pattern: websocket
Action: rewrite 
url: wss://localhost:8069/{R:1}
(use ws:// if not using SSL)
type: permanent

Rule #2 (below #1): ReverseProxyHTTP
regular expression: ^(.*)$
Action: rewrite
url: http://localhost:8069/{R:1}

odoo.conf
proxy_mode = True

not entirely if really necessary in this context:
netrpc_interface = 127.0.0.1
xmlrpc_interface = 127.0.0.1

xmlrpc_port = 8069

Hope it helps someone.
Video works fine too, but only when I use Twilio.