socket.io v. 3: socket.io-client-swift cannot connect using allowEIO3

[andreassemborg]

Hi,
I am struggling with the transition to socket.io v.3. I really appreciate the allowEIO3 option and it works great for my web based clients. However, when a swift client tries to connect, I get the following error:

socket.io-client-swift log:

2021-02-28 10:23:23.273062+0100  Chat[18272:5342173] ERROR SocketEngine: connection closed by server. code=1000, type=protocolError
2021-02-28 10:23:23.273347+0100 Chat[18272:5333364] ERROR SocketManager: connection closed by server. code=1000, type=protocolError
2021-02-28 10:23:23.273452+0100 Chat[18272:5333364] LOG SocketIOClient{/}: Handling event: error with data: ["connection closed by server. code=1000, type=protocolError"]
2021-02-28 10:23:23.280708+0100 Chat[18272:5333364] LOG SocketIOClient{/}: Handling event: reconnect with data: ["connection closed by server. code=1000, type=protocolError"]
2021-02-28 10:23:23.352060+0100 Chat[18272:5342177] ERROR SocketEngine: connection closed by server. code=1000, type=protocolError

socket.io log:

socket.io:server initializing namespace / +0ms
socket.io:server creating engine.io instance with opts {"allowEIO3":true,"path":"/socket.io"} +1ms
socket.io:server attaching client serving req handler +5ms
socket.io:server incoming connection with id 4y-oIhowKx6yCO4kAAEl +49ms
socket.io:client client close with reason transport close +66ms

Versions used:
socket.io-client-swift: 13.1.0
socket.io: 3.1.2

The swift client can connect when server is running socket.io 2.4.0.

A bug or a misconfiguration on my end?