In den Programmoptionen einstellen:

builder.Services.AddControllersWithViews()
         .AddNewtonsoftJson(options => { 
         options.SerializerSettings.ReferenceLoopHandling = Newtonsoft.Json.ReferenceLoopHandling.Ignore;
         options.SerializerSettings.DateFormatString = "yyyy-MM-dd";
         }
     );