Why am I getting the error SlingAuthenticator handleLoginFailure

Why am I getting the error SlingAuthenticator handleLoginFailure: Unable to authenticate: UserId/Password mismatch?

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There could be a couple of reasons causing this issue. Kindly check for the below details:

1. Check to see if the user is correctly mapped in AEM.
2. See if correct Active Directory attributes are mapped in AEM.
3. See if it is happening for one user or all the users.
4. See the crx/quickstart logs for any entries.

In addition, to get a better understanding of AEM secure sites, read Building Secure Adobe Experience Manager Web Sites. 

Also, it is recommended that you also watch the following Ask the AEM Community Experts: Building Secure AEM web sites.