If I am not mistaken, joplin server does currently not support starttls for LDAP. This would however be great since it is the de facto standard for smaba AD domain controllers to secure LDAP binds via starttls. The underlying library ldapts would support this feature (Implement StartTLS · Issue #71 · ldapts/ldapts · GitHub).
If I (or someone else interested for that matter) would have time to implement this, would a merge request be accepted?