Understanding an error message

I'm trying to follow the instructions at https://blog.joinmastodon.org/2018/06/how-to-implement-a-basic-activitypub-server/

I get the error message: {"error":"Unable to fetch key JSON at https://<domain>/actor"}

The actor file is located at: /var/www/<subdomain>/actor

that contains:

{
    "@context": [
        "https://www.w3.org/ns/activitystreams",
        "https://w3id.org/security/v1"
    ],

    "id": "https://<domain>/actor",
    "type": "Person",
    "preferredUsername": "alice",
    "inbox": "https://<domain>/inbox",

    "publicKey": {
        "id": "https://<domain>/actor#main-key",
        "owner": "https://<domain>/actor",
        "publicKeyPem": "-----BEGIN PUBLIC KEY-----\nMIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8AMIIBCgKCAQEAnioC3W5cHbHLo+ln4QPm\n6EX1Ypvs5h+h47b5RwhF+2N84dSHXURUuA6a33+IlVqiO/3gGxIX+4MJuSDAxGZM\nWFDbuqDnHft2zWorKmIqyDzSSB/oMSqOVL8QV1KYWsCyhw9v2l5U6EIw2Z7u3Fc9\nW19dumK0ieL7CPME2dJGV5quBoa2AGg0D36XCp8hAWMXGUnlEcaqjyePJg9/quZn\no3W/+h5ujoUQG73SIUaJdsK16Q0E7o5N7JP7pqqE8p8pEXtpwXJJYE9y9ItTqvRS\na29xymyboCI6jSK+VTgjz1tj6W+EGQsPfka6t8Gq82NL00VxBEYb/eiEFuOSy4RL\ndwIDAQAB\n-----END PUBLIC KEY-----"
    }
}

I haven't been able to find out what that error means. Is it unable to find the actor file? Maybe I should change /actor to /actor/ and put the file inside that? If so, what should the filename be? Is it some problem with the public key inside the JSON? Is it mad about the newlines?