Telegram Bot API - Webhook

Question

I'm trying to set a webhook for my inline bot.

The server is hosted by Google Compute Engine, running Windows Server 2016, IIS 10, is bound to a domain and have a self-signed certificate (I think that's the part that is blocking me)

I created a Web API in C# and tested it with Postman.

I've checked out this Telegram page and this answer, but I'm still having problems.

Return of https://api.telegram.org/botXXX/getWebhookInfo:

{
    "ok": true,
    "result":
    {
        "url": "https://myserver.url:443/path/to/api",
        "has_custom_certificate": true,
        "pending_update_count": 41,
        "last_error_date": 1523561516,
        "last_error_message": "SSL error {337047686, error:1416F086:SSL routines:tls_process_server_certificate:certificate verify failed}",
        "max_connections": 100
    }
}