Note that X509 is a certificate type, so yes this error is related to QlikClient certificate. Are you having a larger issue with this environment or did you see this error and are looking for clarification? It could be the error was returned due to a brief lapse in communication between the services. If there isn't a larger problem, the error is likely benign. Hope this helps!
Sr. Technical Support Engineer with Qlik Support Help users find answers! Don't forget to mark a solution that worked for you!