The OPC will check to see if your configured SSL certificate is about to expire, this alert has three levels of warning.

Low 30 days Medium 14 days High 7 days

If you are using LetsEncrypt the time scales are shorter:

Low 14 days Medium 7 days High 3 days

The OPC will try to automatically renew the SSL certificate if you are using a self-signed option. You can also manually run a certificate update from within the OPC

Go into Settings > Host Settings and click on Renew Certificate

Once updated, the problem alert will clear the next time the scheduled check is ran.