ESP32 over HTTPS: can't get valid certificate

Hi guys!

I have recently been using ESP32; my application will send data to a server over https; i start to see the WiFiClientSecure example that use the WiFiClientSecure library.
I have difficult to extract the root CA certificate from my server: the obtained certificate not work; moreover if I try to extract the root CA from the example server (www DOT howsmyssl DOT com), the obtained certificate is different from that of the example.

Any ideas?
Thanks in advance!
Mauro