After the recent Windows 11 update KB5066835, many developers found their local web servers — including IIS, IIS Express, and Kestrel — suddenly failing to load. Browsers began throwing ERR_CONNECTION ...
Update 10/17/25: Microsoft fixes the bug using a KIR (Known Issue Rollback) update. More information added to end of story. Microsoft's October Windows 11 updates have broken the "localhost" ...