A second operation started on this context before a previous operation completed

The exception tells that db context is being used by two threads at the same time; either two threads in the same request, or by two requests. As you mention that you are execute separate thread for long process.

This error in some cases maybe caused by following scenario: if we call an async method but did not used await before calling the method. my problem solved by adding await before the method. however the answer might not be related to the mentioned question but it can be help to similar error.

You can use lock statement to handle concurrency (https://docs.microsoft.com/en-us/dotnet/csharp/language-reference/keywords/lock-statement).

lock (x)
{
  // Your code...
}

In your case, I think, as you mentioned that you are calling from controller action so might be second request came before completion of fist one. In this case following code will resolve your problem:

private readonly object lockObject = new object();

new Task( () =>
{
  lock (lockObject)
  {
    // long runing process and fetching data using EntityFrameworkCore
  }
}).Start();

Any other thread will be  blocked from acquiring the lock and waits until the lock is released.

I am not sure if you are using Dependency Injection to resolve your DbContext in your .net core application. If you are using native IoC from .NET Core (or any other IoC-Container) and you are getting this error, ensure to register your DbContext as Transient as:

services.AddTransient<MyContext>();
//OR
services.AddDbContext<MyContext>(ServiceLifetime.Transient);
//Instead Of
services.AddDbContext<MyContext>();

AddDbContext or AddScoped adds the context as scoped, which may be cause troubles when working with multiple threads.

Also async / await operations can cause this behavior, when using async lambda expressions.

Adding db context as transient also has its drawbacks. You will not be able to make changes to some entity over multiple classes that are using the context because each class will get its own instance of your DbContext.