Actually the error message from the original post has a different reason:
The target system image had probably just been applied from the service stick. The RAT service on the PLC creates its own certificate when it is started the first time. This certificate has a period of validity which starts at the time where it is created.
The realtime clock of some PLCs (e.g. C6017) tends to run a little fast. I.e., at the time it is created, the certificate is not yet valid when evaluated on a computer where the system time is not ahead.
And that's the error message you got: The development computer (client) does not accept the certificate from the PLC (server) because it is not yet valid. So actually, this problem was solved by waiting for some minutes, not by the steps in the accepted answer.
Nevertheless, the hint in the accepted answer is correct: When deploying to the target system for the first time, you need to trust the client certificate on the server side. Otherwise you also get an error message, though it would be this one:
RAT client: Secure connect failed. Make sure the client certificate is in the servers "trust" list.
maj9bj describes correctly how the certificate can be trusted manually on the file level. Alternatively, you can use the SecurityManager app on the target system which is accessible through the tray icon looking like a shield.
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