What finally worked was adding this to the composer.json file:

{
"repositories": [
    {
        "type": "composer",
        "url": "https://repo.packagist.org",
        "options": {
            "ssl": {
                "verify_peer": false,
                "allow_self_signed": true,
                "cafile": "D:/xampp_7_4_33/apache/bin/curl-ca-bundle.crt",
                "capath": "D:/xampp_7_4_33/apache/bin"
            }
        }
    }
]
}

This solution was found in this post: Composer Curl error 60: SSL certificate problem: unable to get local issuer certificate (search for "Composer Specific Solutions").