Microsoft has acknowledged an issue affecting Windows Server 2016 systems related to domain controller lookups after the installation of the KB5087537 security update released in May 2026. The problem occurs specifically for devices with hostnames that are exactly 15 characters long, causing domain controller discovery to fail and resulting in an ERRORINVALIDPARAMETER during DCLocator calls. This issue may disrupt administrative operations that depend on domain controller lookups, such as DFS Namespace management. Microsoft is investigating the issue but has not provided a timeline for resolution.