I had this issue when I placed my Socket.IO server behind cloudflare. Cloudflare is used to protect servers from attacks and acts like a proxy between the user and your server. The problem here seems that cloudflare was having issues proxying to the socket.io. My guess is that cloudflare was trying to connecto the socket.io server using http and not https and this is why these 404 not found errors were happening. This video explains how to fix this issue. Maybe most people are not implementing SSL on their socket.io servers and not experiencing this problem but it's safer to implement SSL on both cloudflare AND your Socket.IO server.

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