X_WEBHOOK_RECEIVER_INVALID
Means
a webhookReceiver() declaration cannot be served
Typical cause
the path is not absolute, sits under /_x or /api (the framework serves both), or a field the receiver needs is missing or not a function (verify, handle, a declared handshake), or maxBytes is not a whole number of at least 1. Refused at declaration, while modules load: no request is ever answered with it
Fix
fix the field the cause names in webhookReceiver("<name>") — path: "/hooks/<vendor>", verify and handle as functions