VAST：AdError 1012：由于网络错误，无法从服务器请求广告

Question

在使用PHP-Vast来测试VAST时出现错误，例如“广告错误：AdError 1012：由于网络错误而无法从服务器请求广告。” 在测试站点： https://developers.google.com/interactive-media-ads/docs/sdks/html5/vastinspector

XML生成结果：

<?xml version="1.0" encoding="utf-8"?>
<VAST version="2.0">
    <Ad id="MyAdsVast">
        <InLine>
            <AdSystem>
                <![CDATA[Ad Myads Vast]]>
            </AdSystem>
            <AdTitle>
                <![CDATA[MyAds Video Vast]]>
            </AdTitle>
            <Impression id="imp1">
                <![CDATA[http://example.com]]>
            </Impression>
            <Creatives>
                <Creative>
                    <Linear>
                        <Duration>00:00:18</Duration>
                        <VideoClicks>
                            <ClickThrough>
                                <![CDATA[https://example.com/]]>
                            </ClickThrough>
                            <ClickTracking>
                                <![CDATA[https://example.com/]]>
                            </ClickTracking>
                            <CustomClick>
                                <![CDATA[https://example.com/]]>
                            </CustomClick>
                        </VideoClicks>
                        <TrackingEvents>
                            <Tracking event="start">
                                <![CDATA[https://example.com/]]>
                            </Tracking>
                            <Tracking event="pause">
                                <![CDATA[https://example.com/]]>
                            </Tracking>
                        </TrackingEvents>
                        <MediaFiles>
                            <MediaFile delivery="progressive" type="video/mp4" height="100" width="100" bitrate="2500">
                                <![CDATA[https://example.com/MyAds_Video1.mp4]]>
                            </MediaFile>
                        </MediaFiles>
                    </Linear>
                </Creative>
            </Creatives>
        </InLine>
    </Ad>
</VAST>

然后在根服务器（public_html）中创建crossdomain.xml

<?xml version="1.0"?>
<cross-domain-policy>
    <allow-access-from domain="*" secure="false" />
</cross-domain-policy>

所有代码正常工作。请给我建议。谢谢