Received HTTP/0.9 when not allowed

I stumbled over a weird behavior when I try to send a post HTTP/2.0 request to apples push service:

        $http2ch = curl_init();
        curl_setopt($http2ch, CURLOPT_HTTP_VERSION, CURL_HTTP_VERSION_2_0);
        curl_setopt($http2ch, CURLOPT_URL, 'https://api.push.apple.com/3/device/megauniquedevicetokendummy');
        curl_setopt($http2ch, CURLOPT_PORT, 443);
        curl_setopt($http2ch, CURLOPT_HTTPHEADER, $httpHeader);
        curl_setopt($http2ch, CURLOPT_POST, true);
        curl_setopt($http2ch, CURLOPT_POSTFIELDS, $body);
        curl_setopt($http2ch, CURLOPT_RETURNTRANSFER, true);
        curl_setopt($http2ch, CURLOPT_TIMEOUT, 30);
        curl_setopt($http2ch, CURLOPT_HEADER, 1);

       $result = curl_exec($http2ch);
       if ($result === false) {
           throw new \Exception("Curl failed: " . curl_error($http2ch) . " | " . curl_getinfo($http2ch, CURLINFO_HTTP_CODE));
       }

The exception is thrown with the Message: Curl failed: Received HTTP/0.9 when not allowed | 0

I explicitly told curl to use HTTP/2.0 on the second line of the code snipped above. Does anyone have any idea what that error message means and why curl uses such an old HTTP version?

I am on PHP 7.2 and curl version 7.66.0.