Microsoft has acknowledged an issue with the May 2026 security update for Windows Server 2016, affecting systems with hostnames of exactly 15 characters, which leads to failed domain controller (DC) lookups. The error occurs during DC lookups, specifically returning the error code ERRORINVALIDPARAMETER. Servers with 14 or 16 character hostnames are not affected. Administrators may face challenges with DFS Namespace management and other functions reliant on DC access. Microsoft is investigating the issue but has not provided a timeline for a fix. Windows Server 2016's mainstream support ended in January 2022, but extended support will continue until January 2027.