HTML5 Browser client can't connect to dedicated server

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I’m trying to connect a browser html5 client to a dedicated server. Connecting to the server works as a standalone client but not from a browser.
I’m using Firefox 62.0.2 to run the game.
The web console returns this when trying to connect.

Edit: It seems like the /Script/HTML5Networking.WebSocketNetDriver isn’t working. The dedicated server prints this when trying to use the WebSocketNetDriver as the only net driver:

CreateNamedNetDriver GameNetDriver already exists as WebSocketNetDriver_0
NetworkFailure: NetDriverCreateFailure, Error: ''

Is the HTML5Networking Net Driver supported at all? Or is only not working on MacServer platform?