The issue occurs because of one of the following:

• The Active Directory server does not have SSL enabled. To use SSL-based access methods, SSL must be enabled, which is often not the default condition. Port 636 is used for SSL connections.
• There is a change in the certificate or it is expired. To resolve the issue:
  1. Log in to DSM using the default account "MasterAdmin".
  2. Go to Administration > User Management > Users.
  3. Click Synchronize with Directory.
  4. Select Add/Update Certificate, and then click Next. The DSM will now try to synchronize with Active Directory to update the certificate.
  5. Click Accept and click Next.
  6. Log out of the DSM and then log in using the Active Directory account.
• In a specific instance, a customer's domain controller was in a different location. The issue was fixed by putting the domain controller in the same subnet as the Deep Security Manager.