Update to this topic, as I didn't find a solution :
- I exported our local CA root certificate with format Base-64 encoded X.509 (.CER) and imported it to Nedi
- To do so : copy the file to /usr/local/share/ca-certificates then run update-ca-certificates
- reconfigure ldap to ldaps (in nedi.conf change path from ldap:// to ldaps:// and port from 389 to 636)