I've managed to fix the problem.

Due to the circular reference inside of Entity Framework the JSON failed to parse.

I manged to fix it by adding this option inside the startup.cs:

 services.AddMvc()
            .AddJsonOptions(options =>
            {
                options.SerializerSettings.ReferenceLoopHandling = Newtonsoft.Json.ReferenceLoopHandling.Ignore;
            });