If the certificate from the controller for encrypted communication has expired ( valid from "not before" until "not after"), you get a prompt with a corresponding message in CODESYS when you attempt to access the controller. For example, to renew the expired certificate, you can accept the expired certificate and connect to the controller.
You will see this message again every time you try to login until a valid certificate is installed on the controller.
If you have created or imported a new certificate on the controller, then this new certificate will be available for you to accept the next time you login.
Other clients that communicate encrypted with the controller (for example, PLCHandler) will typically not accept an expired certificate. This means that no connection can be established here.