SSL certificate is not valid when the server is connecting itself

Certificate chain
 0 s:CN = local.mydomain.com
   i:C = US, O = Let's Encrypt, CN = R3

The server is misconfigured and only sends an incomplete chain, missing the intermediate certificates. A properly configured site should instead return an additional intermediate certificate, leading to ISRG Root X1. Let's Encrypt has an example server for this:

$ openssl s_client -connect valid-isrgrootx1.letsencrypt.org:443
...
Certificate chain
 0 s:CN = valid-isrgrootx1.letsencrypt.org
   i:C = US, O = Let's Encrypt, CN = R3
 1 s:C = US, O = Let's Encrypt, CN = R3
   i:C = US, O = Internet Security Research Group, CN = ISRG Root X1

When using another computer ... Chrome shows that certification is valid.

Browser will usually successfully work around such misconfiguration by downloading the missing intermediate certificates or using cached copies. Other clients will not. So it's not a good idea to only check if it works with the browser. Use instead sites like SSL Labs which would report "chain issues" as problem in your case.