What does different response to ssl requests to different servers mean?

Question

Digital certificates

We make two requests to different servers from under ubuntu:
openssl s_client -connect server1.com:443
openssl s_client -connect server2.com:443
For the first server:

SSL handshake has read 2089 bytes and written 449 bytes
Verification error: unable to verify the first certificate
...
SSL-Session:
...
 Verify return code: 21 (unable to verify the first certificate)

The second one:

SSL handshake has read 3227 bytes and written 438 bytes
Verification: OK
...
SSL-Session:
...
 Verify return code: 0 (ok)

Both servers have letsencrypt certificates. What is wrong and on whose side?

Answer 1

look at the certificates issued by the server ( -showcerts).