NICE DCV behind Nginx reverse proxy, "Failed to communicate with server."

I have Nginx running on an Amazon Linux EC2 instance. It is listening for connections to https://dcv01.example.com and then using proxy_pass with a wildcard cert to serve the DCV client from an internal IP address. The page displays properly with no SSL errors. However, instead of the login prompt, I get a red message that says "Failed to communicate with server.".

I attempted to use the allowed-http-host-regex with the value ^(www\.)?dcv01\.example\.com$, but then I get a 403. I also tried allowed-ws-origin-regex, but it does nothing different (still get Failed to communicate...)

What do I need to do to get DCV working behind a reverse proxy?