This would suggest your n8n instance can’t receive any external data. Does the URL you are seeing the 404 on have the format {adjective}-{noun}-{number}.hooks.n8n.cloud by any chance?
If so, the tunnel opened by n8n to receive data from external services might have previously been interrupted. You might want to try the approach suggested by @ivov in this thread to request a new (and working) subdomain: