We need to see the actual requests hitting the wire. What does Fiddler Show? Also not sure if the above is C# looks like it. We have a nuget that deals with some of the “epicor stuff” for you and abastracts a lot of the pain of calling rest.
Maybe worth a shot… if you are struggling, it even has visbility onto the RAW calls (ala Fiddler) right on the Class.
EpicorRest.AppPoolHost = "your.tld.server";
EpicorRest.AppPoolInstance = "yourEpicorInstance";
EpicorRest.UserName = "epicor";
EpicorRest.Password = "epicor";
EpicorRest.IgnoreCertErrors = true;
//get
Dictionary<string, string> dic = new Dictionary<string, string>();
dic.Add($"filter", "ABCCode eq 'A'");
dynamic abcData = EpicorRest.DynamicGet("Erp.BO.EmpBasicSvc", "List", dic);
//GetJSON String
string json = EpicorRest.DynamicGetJSON("Erp.BO.EmpBasicSvc", "List", dic);
More info at