WebSocket
1010 Mandatory Extension
The client is terminating the connection because the server did not negotiate one or more expected extensions in the handshake response.
When You See This Code
The client requested a required WebSocket extension (e.g., permessage-deflate compression) during the handshake, but the server did not include it in its response.
How to Fix
Enable the required extension on the server, or update the client to make the extension optional. Check the Sec-WebSocket-Extensions header in the handshake response.