15‑character hostnames break domain controller lookups on Windows Server 2016 after May security update
Microsoft has confirmed a new known issue for Windows Server 2016 after the May 2026 KB5087537 security update: domain controller discovery can fail when the server hostname is exactly 15 characters, with DCLocator returning ERROR_INVALID_PARAMETER and potentially blocking admin tools and DFS Namespace tasks. Microsoft is investigating and has not provided a timeline for a fix.