As per the question and summary of the post, I understand that, When the QC authentication is changed to LDAP, the users are now authenticated against the
LDAP server's domain users. As the Quality Center ''Admin'' user is internal to QC, this user will not exist in the LDAP domain, as such this user will not be
able to log in to any Quality Center/ALM application.

The site admin used to install ALM is QUALITYCENTER authentication unless it was established within their LDAP system, if not established then it would be
using QUALITYCENTER authentication. ALM can only use one form or the other and not both. Once they have been able to access the system in site admin they
can go to the Site Users tab and under the red user icon they can add designated users to the site admin user group and when they enable LDAP again then
those users will have access to the site admin. If they choose they can have a site admin specific user added to the LDAP system for accessing site admin
exclusively and they would then need to add that user to the user list through the LDAP import function.

In order to continue to administrate Quality Center, ensure that a user that exists in the LDAP domain has been designated as the site administrator and/or
project administrator before switching to the LDAP authentication. I am suggesting you have to switch the authentication mode.
Below are the steps to switch the authentication mode:

1. Access qcsiteadmin_db schema;
2. Select the PARAMS table;
3. Change the PARAM_VALUE from ''QualityCenter'' to ''LDAP'' for the PARAM_NAME ''AUTHENTICATION'';
4. Restart the QC/ALM service or reboot the server machine.

Note: If the database schema cannot be accessed, please consult with the responsible DB Administrator

UPDATE PARAMS
SET PARAM_VALUE='Ldap'
WHERE PARAM_VALUE='QualityCenter'

Let me know if this resolves the issue.