Solution

1) Make sure the current date/time is set correctly. (Verify in System Status)

2) Renew / Replace the current security certificate with a new certificate prior to the expiry of the current certificate.

If the IP Office default certificates are being used, they can be regenerated from either Manager [in the security settings] or Web Manager.

Below shows how to view the certificate in Manager > File > Advanced > Security settings;

More:  https://medium.com/tekstore/ip-office-manager-shows-a-warning-the-security-certificate-will-expire-in-xxx-days-96b6fee7fdc6