Websockets Proxy: Client to server connection dies

[UltraBlackLinux]

Hey there,
I'm currently trying to proxy HTTP Websocket connections between a client and vscode-server, but the websocket requests are always "interrupted while the page was loading". I really don't know why exactly this is happening. I went ahead and spun up the example websockets server, but it talked fine through my proxy.

And here is what vscode-server is doing:

client -> server (length=104, flags=0x82).
server -> client (length=103, flags=0xc2). # what on earth is c2?? It's not documented anywhere
client -> server (length=0, flags=0x0).

Do you reckon there is anything that can be set before communication starts? I'm out of ideas and I just cannot explain this to myself. It's not helping that I have zero prior websockets experience.
Maybe you have any other idea?
Thanks!