ASP.NET Core : Synchronous operations are disallowed. Call WriteAsync or set AllowSynchronousIO to true instead

You might have the following problem: https://github.com/aspnet/AspNetCore/issues/8302

And you can find more info here: https://github.com/aspnet/AspNetCore/issues/7644

A workaround until the issue is being solved is to allow Synchronous IO. Put this in Startup.cs for either Kestrel or IIS:

public void ConfigureServices(IServiceCollection services)
{
    // If using Kestrel:
    services.Configure<KestrelServerOptions>(options =>
    {
        options.AllowSynchronousIO = true;
    });

    // If using IIS:
    services.Configure<IISServerOptions>(options =>
    {
        options.AllowSynchronousIO = true;
    });
}

I had this issue with my unit tests. I had to update my TestServer to AlloSynchronousIO

Server = new TestServer(new WebHostBuilder().UseStartup<Startup>());
Server.AllowSynchronousIO = true;