X_WEBHOOK_ENDPOINT_INVALID
Means
the endpoint cannot be delivered to as declared
Typical cause
three conditions, one code, and cause names which: an empty secret, a url that is not a url, or a scheme that is not http: or https:. The scheme is refused by NAME rather than by a blocklist — a delivery opens an HTTP conversation, and file: or data: is a row in a table reaching the worker's filesystem. A delivery with no secret is an unsigned POST to wherever the row points
Fix
give the endpoint an https:// url and a non-empty secret before webhook("<name>").endpoint returns it