Windows Server 2016 Domain Controller May Fail with 15-Character Hostname

A May 12, 2026 Microsoft cumulative update (KB5087537) introduced a bug on Windows Server 2016 that breaks domain controller discovery for servers whose hostnames reach the 15-character NetBIOS limit: DCLocator now returns ERROR_INVALID_PARAMETER, disrupting authentication, Group Policy, DFS Namespace management and other Active Directory–dependent services; Microsoft acknowledged the issue on May 22, 2026 and recommends renaming affected servers as a temporary mitigation while a fix is developed.