Email groups not returning with campaignFolders



We are using PerceptiveMCAPI dll to make API calls to mailchimp and getting the data in to our system

Recently we are facing one issue with the method campaignFolders method, even though In the mail chimp interface account we are sorted the emails by creating folders, but the below code is not returning folders it is showing count as 0,
                // Getting the email group dimensions from the provider
                campaignFoldersInput input = new campaignFoldersInput(ApiKey);
                campaignFolders folders = new campaignFolders(input);
                campaignFoldersOutput output = folders.Execute();
This is happening only with one specific account, and getting this error
response contains string value where array expected [response]

at CookComputing.XmlRpc.XmlRpcSerializer.ParseString(XmlNode node, Type ValueType, ParseStack parseStack, MappingAction mappingAction)
at CookComputing.XmlRpc.XmlRpcSerializer.ParseValue(XmlNode node, Type ValueType, ParseStack parseStack, MappingAction mappingAction, Type& ParsedType, Type& ParsedArrayType)
at CookComputing.XmlRpc.XmlRpcSerializer.DeserializeResponse(XmlDocument xdoc, Type returnType)
at CookComputing.XmlRpc.XmlRpcSerializer.DeserializeResponse(Stream stm, Type svcType)
at CookComputing.XmlRpc.XmlRpcClientProtocol.ReadResponse(XmlRpcRequest req, WebResponse webResp, Stream respStm, Type returnType)
at CookComputing.XmlRpc.XmlRpcClientProtocol.Invoke(Object clientObj, MethodInfo mi, Object[] parameters)
at XmlRpcProxy83a76e98-c32b-4aa6-a284-1f414eec265d.campaignFolders(String apikey)
at PerceptiveMCAPI.Methods.campaignFolders.ExecuteXmlRpc()

We have other accounts and are able to retrieve the email groups with out any issues, the above same method to retireve the email folders is working for other accounts in mail chimp.