The "Certificate Expired" message can mean any of the following:
- The date, time and/or year in your device are incorrect.
- You need to update your device's software to the latest available version.
For information on how to update your device software with Nokia, please visit Nokia's website.
If you are using a Sony Ericsson phone and you are still having this problems after checking the above, unfortunately this is a known issue and there is nothing we can do to help at this moment.
WhatsApp Support Team