Last week, Microsoft issued a cumulative update for Windows 11 (KB5066835), which unexpectedly disrupted the HTTP/2 connectivity to the local host (localhost/127.0.0.1). This glitch hindered developers from effectively debugging their applications in local development environments. The root cause was traced to a clash between the HTTP.sys kernel component and the update, leading to connection resets (ERR_CONNECTION_RESET) and protocol errors (ERR_HTTP2_PROTOCOL_ERROR). These issues had a significant impact on ASP.NET compilation, Visual Studio debugging sessions, and any processes dependent on IIS authentication.

Developers affected by this problem were compelled to uninstall both KB5066835 and the September preview update KB5065789 to regain normal functionality. Recognizing the severity of the situation, Microsoft has promptly acknowledged the issue and is currently working on a comprehensive fix.