We had the same issue on only the machines that had that KB5003637 installed. We uninstalled it and everything was fine. We decided to upgrade to .net 5 in the end which fixes the issue as it was fairly painless for our projects.

If you can't upgrade try disabling HTTP/2

serverOptions.ConfigureEndpointDefaults(lo => lo.Protocols = Microsoft.AspNetCore.Server.Kestrel.Core.HttpProtocols.Http1);

Mine looks like this for .net core 3.1

public static IHostBuilder CreateWebHostBuilder(string[] args) =>
            Host.CreateDefaultBuilder(args)
                .ConfigureWebHostDefaults(webBuilder =>
                {
                    webBuilder.ConfigureKestrel(serverOptions =>
                    {
                        serverOptions.Limits.MaxRequestBodySize = int.MaxValue;
                        serverOptions.Limits.MinResponseDataRate = null;
                        serverOptions.ConfigureEndpointDefaults(lo => lo.Protocols = Microsoft.AspNetCore.Server.Kestrel.Core.HttpProtocols.Http1);
                    })
                    .UseStartup<Startup>();
                });
        }

this issue (on iis express) on only the OS that had that KB5003637 installed. if you unable delete this update first upgrate windows 10 to last by https://www.microsoft.com/en-ca/software-download/windows10 then you can delete kb5003637

(don't forget disable windows auto update)

I got this error creating a new blazor app vs2019/windows10 update to latest versions. After looking at a prior working blazor project Startup.cs I fixed by commenting out //app.UseHttpsRedirection(); I think this maybe a new project template problem

Disable the SSL from the project properties it will automatically down to http/1 and then enable the javascript debugger for the browser

Hope this will help

I tried everything including rolling back updates and a full reinstall of Windows. Finally, a Microsoft support rep posted a workaround for this that involves disabling HTTP2. This seems to have worked for me as a temporary solution. In summary:

1. Start the Windows Registry Editor
2. Navigate to the registry key HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\HTTP\Parameters
3. Add 2 new REG_DWORD values, EnableHttp2Tls and EnableHttp2Cleartext, to this registry key
4. Set both values to 0
5. Reboot the machine

The rep notes:

The registry values disable HTTP/2 on the machine. You can remove those values when the fix to https.sys is published.

In my case, upgrading the project to .Net core 5.0 (as @SpeedofSpin suggested) did not help - I had the same issue.

Disabling SSL (as @samejeep suggested) for application was demanding, because my projects uses IdentityServer4 and it just stopped authenticating after that - so I didn't continue that path.

But as a workaround I have changed all requests for css, js, and static files via non-SSL http:// requests.

So for example, instead of this

<link rel="stylesheet" href="~/css/site.css">

Which would use the current request's protocol https://, I've used absolute paths, e.g.:

<link rel="stylesheet" href="http://localhost/css/site.css">

Note, that I'm stating "http" (not "https") explicitly.

I did that for css styles and js scripts and that was enough for me. I didn't have to do it for images though.

For me this is just a dirty workaround (I'll be waiting for MS to release a patch), but at least it allows me to keep doing my work.

Update:

I've found a better way to disable HTTP/2 via registry on my localhost and that solves the problem too.

I found that disabling SSL (which I think results in IIS Express dropping down to HTTP 1.1) is a workaround.

In my case it was ASP.NET Core Identity that caused the trouble: I had too many roles (user claims) which were stored as session cookies in my browser and caused a "HTTP 400 Bad Request - Request header too long" error. This was showing as HTTP2 protocol error in Chrome and SSL certificate error in Firefox since I was using HTTPS redirection.