godot: mbedtls error when using private key on server

Godot version: Godot Engine v3.2.1.stable.official - https://godotengine.org , server build OS/device including version: Ubuntu 18.04.4 LTS Issue description: Console prints mbedtls error: returned -0x7780 and connection is not established Steps to reproduce: Using the server build on linux try to use a private key from LetsEncrypt , then have a godot client connect. Minimal reproduction project:

extends Node

var server: WebSocketServer = null

func _ready():
    server = WebSocketServer.new()
    server.private_key = load("res://privkey.key")
    server.ssl_certificate = load("res://fullchain.crt")
    server.listen(port, PoolStringArray(), true)
    get_tree().set_network_peer(server)

func _process(delta):
    if server.is_listening():
        server.poll()