Linux server with Unity Networking and Websockets not working

I have created a small multiplayer test game with Unity Networking (version 5.6 for Windows). My objective was to have the client in WebGL and the server running as Linux standalone (headless).

To do that I need to use websockets. But, after testing lots of combinations, that configuration is still not working. The WebGL client cannot connect with the Linux server.

So far, I don't have any issue with:

Editor server (websockets) - WebGL client Windows server (websockets) - WebGL client Linux server (no websockets) - Windows client (no websockets)

So it seems that the Linux server is not accepting websockets connections. I have made a capture of the frames and the server accepts the TCP connection from the client but then closes it instead of starting the Websockets protocol.

I am sure that the server is configured to use Websockets because, in that configuration, it doesn't accept connection from the Windows client (with no websockets).

From my Google search, I see that this combination is working for some people. So maybe is my Linux version (Ubuntu 16.04 64-bit) or maybe there is a bug in Unity 5.6 that didn't exist before.

Do you have any experience with this configuration in Unity 5.6?

Thanks.