Login Error in iOS/Android SDK

I have integrated mobile iOS SDK.

Initialise SDK with : 

Zendesk.initialize(appId: "my_app_id", clientId: "my_client_id", zendeskUrl: "my_url")

Support.initialize(withZendesk: Zendesk.instance)

Then set identity : 

let identity = Identity.createJwt(token: userId)

Zendesk.instance?.setIdentity(identity)

In console it shows identity has been set.

But whenever I tried to create ticket it says :

[ERROR] : Login error : Error Domain=com.zendesk.sdk.core Code=1000 "unauthorized" UserInfo={error=Couldn't authenticate you, HTTP Status code=401, NSLocalizedDescription=unauthorized}

function to create ticket : 

func buildCreateRequest(callback: @escaping (ZDKCreateRequest) -> Void) {

        let request = ZDKCreateRequest()

        request.subject = subjectTextField.text ?? requestConfig.subject

        request.requestDescription = descriptionTextView.text ?? "Default: The problem of the ticket"

        request.tags = requestConfig.tags

        request.customFields = getCustomFields()

        if(self.attachmentImageView.image != UIImage.init(named: "picture"))

        {

            uploadAttachment { (attachment) in

                if let attachment = attachment {

                    request.attachments.append(attachment)

                }

                callback(request)

            }

        }

        else

        {

            callback(request)

        }

    }

 

Request to create ticket : 

buildCreateRequest {

        ZDKRequestProvider().createRequest($0)  { [unowned self] (result, error) in

            DispatchQueue.main.async {

                VIEWMANAGER.hideActivityIndicator()

                if result != nil {

                    VIEWMANAGER.showToast("Ticket updated successfully")

                }

                if error != nil {

                    VIEWMANAGER.showToast("Failed to update ticket")

                }

            }

        }

    }

 

Please note : above error is not happening with every user. It happens for some random users.