Hi @molcay, I can help answer some of your questions.
A single authentication token is limited to a total of 10,000 active webhooks at any one time, and any given object in Asana is limited to 1000 active webhooks across all apps. When you remove a webhook by calling DELETE /webhooks/<webhook-id> it is deleted immediately as part of the request.
If your requests to create webhooks are being rejected, I’d first verify that you don’t have any active webhooks by calling GET /webhooks. If you haven’t hit the limit of 1000 webhooks, try making a webhook on a different project in case the original project has hit the limit of 1000 webhooks. If you’re still unable to make a webhook, let us know what you observed in these tests so we can better help.