Can you confirm which version of the .NET SDK you are using for the Box V2 API calls? The officially supported version is available at: https://github.com/box/box-windows-sdk-v2
To update the folder upload email access, you can use the following:
BoxFolderRequest folderReq = new BoxFolderRequest()
{
Id = "YOUR_FOLDER_ID",
FolderUploadEmail = new BoxEmailRequest() { Acesss = "open" }
};
BoxFolder f = await _foldersManager.UpdateInformationAsync(folderReq);
Feel free to open issues on the github page should you come across any problems. Thanks!