If you're suddenly unable to log in via your SimpleSAML-based single sign-on, and there's no obvious explanation, and while troubleshooting (where you can find SimpleSAML logs), you see this in the logs:
Jun 24 11:23:46 simplesamlphp ERROR [f1bdf14011] Caused by: SimpleSAML\Error\Exception: Received an assertion that is valid in the future. Check clock synchronization on IdP and SP.
Send a support request to your identity provider and ask them to investigate. Their clock may have drifted from the synchronized world clock. If that does not resolve the issue, file a ticket with Acquia Support and we can investigate on our end (make sure to mention that you've already talked to your identity provider).