This is a very common serialization exception we see when forget to use loop handling strategies in Web API.
Look at the below screenshot which you have noticed:
Now if you want to resolve this issue, you can use following code in web api configuration, here's how you can resolve this issue:
var json = config.Formatters.JsonFormatter;
json.SerializerSettings.ReferenceLoopHandling = Newtonsoft.Json.ReferenceLoopHandling.Ignore;
Here's the complete code snippet screenshot:
Hope this helps you.