I probabely just point out what you already know:
In the QMC/User directory connectors you will have to enter the correct path to the LDAP (most likely in the form of LDAP://yourDomain), the user name of the technical user for your QS Engine (that's the user that you also used as superuser when installing QS) as well as the password for this user. Whenever you make any change/adaptation to this user directory connector, you will need to enter also the password for the tech user - otherwise the connector will not work. Unfortunately, the password is always thrown away and needs to be re-entered.
This technical user will also have to be enabled within your LDAP directory, so that it can access through the network.
The logs Shraddha mentions will somewhere have an entry saying that "Access denied to [tech]user" , "User and password not corresponding" or similar.
this may help you as well?