ASP.NET Core 3.0 [FromBody] string content returns “The JSON value could not be converted to System.String.”

Not sure this help but I think they made some change in .net core 3.0 Newtonsoft.JSON package so you can try this

Install Microsoft.AspNetCore.Mvc.NewtonsoftJson package.

In your startup.cs add

services.AddControllers().AddNewtonsoftJson();