thanks for your patience. The error message you are seeing indicates that the webhook validation handshake failed for your Azure HTTP trigger function. The Handshake can fail if the webhook is not handling the Subscription Validation event. Is this the first time you have seen this error? Is there a way we can see the Azure Function code and your webhook configuration? A screenshot would work.
You can also look at activity logs and see if you can find more information by your activity Id.
WebHook event delivery - Azure Event Grid | Microsoft Learn
Here is more documentation on VNet
Network security for Azure Event Grid resources - Azure Event Grid | Microsoft Learn
We look forward to hearing from you.
Best,
Grace