Try to logon to Active Directory from Shell using ldapsearch.
ldapsearch -x -D "<user>@ads.domain.tld" -b "ou=<ou containing that user>,dc=ads,dc=domains,dc=tld" -H ldap://<AD server name>.ads.domain.tld:389/ -W sAMAccountName=<user>
Test for both, the bind user and the user not working. Both should give back the same result.
Example:
https://community.grommunio.com/d/1558-active-directory-ldap-sync-with-starttls-not-working/4