Webhook call failed: URL_REJECTED error in DialogFlow v2 Fulfillments

(I answer immediatly because we fixed it before posting the question. But I posted nevertheless because it may be useful for others)

The problem was that the webhook was called using http. Setting https solved the problem.

It seems that Google activated a webhook policy of rejecting unsecure calls in their servers. It may have been deployed gradually on their cluster, which would explain the gradual degradation.

We know that we should have migrated to https a long time ago, but still we didn't find any mention of the application of this policy on the net.