Webhook errors can be a real pain to debug since they often fail silently or with very generic messages. I’ve found that most of the time it comes down to either a TLS/SSL handshake issue or the endpoint expecting a specific header that FusionAuth isn't sending.
One thing that really helps is using a tool like Webhook.site or RequestBin just to see if the event is actually firing and what the payload looks like. If it works there but not on your server, it’s almost certainly a firewall or certificate trust issue on your end. Also, double-check that your secret is matching up perfectly - it’s easy for a stray whitespace to throw the whole signature validation off!