I had the similar issue using Flurl.Http:
Flurl.Http.FlurlHttpException: Call failed. Cannot send a content-body with this verb-type. GET http://******:8301/api/v1/agents/**** ---> System.Net.ProtocolViolationException: Cannot send a content-body with this verb-type.
The problem was I used .WithHeader("Content-Type", "application/json")
when creating IFlurlRequest.