Page experience issue

Hello,

When I try to test a amp page in the page experience tool ( https://amp.dev/page-experience/ ) I get:
```
This URL is invalid.
We were unable to load the page you requested. Make sure you are testing the correct URL and that the server is responding properly.
```
But the url is ok if I try google amp test tool: https://search.google.com/test/amp.

When the validation starts I see this message:
```
Please wait a moment
We are currently analyzing your page . . .
0 out of 27 checks are completed
```
But then after 2-3sec I get the error.

I've tried with other AMP pages of my projects and I get that error on all of them.
https://www.animalpedia.it/il-mio-gatto-e-apatico-cause-e-cosa-fare-3748.html?amp=1
https://www.recetasgratis.net/receta-de-arroz-con-leche-integral-75932.html?amp=1

Analyzing the requests received by the web server coming from amp.dev, all of them return 200 http response code.

```
Ip: 35.189.125.240
Request GET /il-gatto-non-mangia-e-vomita-cause-e-cosa-fare-3060.html?amp=1 HTTP/1.1
Status: 200
Referal: https://amp.dev/page-experience/
User agent: Mozilla/5.0 (Linux; Android 6.0.1; Nexus 5X Build/MTC19V) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/51.0.2704.81 Mobile Safari/537.36 (compatible; amp.dev/playground)

```

Thanks!

Reproduce the failure in the AMP Page Experience tool using one of the listed AMP URLs, then compare it with the Google AMP Test result. Inspect the requests from amp.dev and the reported 0-of-27-checks failure; done means identifying why valid AMP pages are rejected or documenting the external service failure.

Written by the indexing model from the issue text.