ldap search fails with Let's Encrypt certificate

[Joel Carnat]

Hi,

I'm using ldap(1) to query a remote Synology Directory Server (OpenLDAP 
2.4.x).
Unfortunately, it fails saying:
  TLS failed: handshake failed: error:14004410:SSL 
routines:CONNECT_CR_SRVR_HELLO:sslv3 alert handshake failure
  ldap: LDAP connection failed
When I use the OpenLDAP ldapsearch, same arguments, I succeeds.

Using openssl s_client, I could confirm that the OpenLDAP server accept 
TLS:
  New, TLSv1/SSLv3, Cipher is AES256-GCM-SHA384
  Server public key is 2048 bit
  Secure Renegotiation IS supported
  Compression: NONE
  Expansion: NONE
  No ALPN negotiated
  SSL-Session:
      Protocol  : TLSv1.2
(...)

Looking inside /etc/ssl/cert.pem, I could find "/O=Digital Signature 
Trust Co./CN=DST Root CA X3".
Which is part of the Let's Encrypt certificate chain.

Is this a known issue or am I missing something?

Thanks.