Please : your exact settings

This follows the bug: 19856

$serverId=0;
$ldap_server[$serverId]['server'] = "ldap.domain.fr";
$ldap_server[$serverId]['port'] = "636";
$ldap_server[$serverId]['protoversion'] = "ldapv3";
$ldap_server[$serverId]['encrypt'] = "ldap";
$ldap_server[$serverId]['referrals'] = false;
$ldap_server[$serverId]['binddn'] = "cn=user,ou=applications,dc=XXXXX,dc=fr";
$ldap_server[$serverId]['bindpw'] = "password";

Can you test with
$ldap_server[$serverId]['encrypt'] = 'ldaps';

It's the way to do, looking at code : i think it don't work, but please : test and return the issue.

absolutely, after testing with ['encrypt'] = 'ldaps' it doesn't work

@ccoupeau : can you test https://github.com/LimeSurvey/LimeSurvey/pull/4217

with $ldap_server[$serverId]['encrypt'] = 'ldaps';

What's happen; no condition met for encrypt == ldaps and protoversion == ldapv3

With $ldap_server[$serverId]['encrypt'] = 'ldaps'; and protoversion == ldapv3 I have this error : Erreur: Ne peut se connecter à l’annuaire LDAP

The only solution for me is :

$ds = ldap_connect("ldaps://" " . "{$ldap_server[$server_id]['server']}:{$ldap_server[$server_id]['port']}");

$ldap_server[$serverId]['server'] = "ldap.domain";
$ldap_server[$serverId]['protoversion'] = "ldapv3";
$ldap_server[$serverId]['encrypt'] = "ldap";

But ?
Do you apply the patch ?

See https://github.com/LimeSurvey/LimeSurvey/pull/4217/files#diff-8eb167bd60eeb3f630dbf81a56c1fb816bfe1fd7ad0b26b4b01834441ad1aeabR32

Then if you set $ldap_server[$serverId]['encrypt'] = "ldaps";
$ds = ldap_connect("ldaps://" . "{$ldap_server[$server_id]['server']}:{$ldap_server[$server_id]['port']}");
is set

Why it don't happen for you ?

You need to apply patch manually : just replace whole function
https://github.com/LimeSurvey/LimeSurvey/blob/dd03213a24b51747ba553817bcfc263c415d2a54/application/helpers/ldap_helper.php#L26

With $ldap_server[$serverId]['encrypt'] = "ldap";

There are no reason this part don't happen
https://github.com/LimeSurvey/LimeSurvey/blob/dd03213a24b51747ba553817bcfc263c415d2a54/application/helpers/ldap_helper.php#L33