Most probable reason for this might be your directory connection configuration,Below is a similar example which i have come across
1)I have created a directory connection,The directory connection was succesfully created and tested.
2) I have configured the AD authentication also for the Same.
3) Checked with Kinit, No error messages being displayed.\
4) created a user in DLP with the AD login name(exactly the same,case sensitive)
5) I tried logging in with this AD user created in the DLP console, but i am facing an error saying invalid username or Password
Note: I have two domains, a parent domain and a child domain. i have created a DLP user who is a part of the parent domain.
I have created the Directory connection using the credentials of a user present in the child domain, so when i am trying to login to the DLP console using the AD user present in the parent domain i am not able to do so.
I have changed the user credentials of the directory connection from the child domain user to parent domain user and now i am able to login successfully.
You can take the help of your AD admin who will help you the best in this regard.
You must ensure that both the DLPusers and the user ID used to create the directory connection are in the same domain.