Symptoms

Confluence is presenting the following error message in the dashboard and/or in some pages:

The license could not be verified: License Certificate has expired!

Cause 

While the error appears to suggest that the Confluence license is expired, it actually comes from a third party plugin (ServiceRocket plugins most likely), so the error is misleading.

Resolution

Check the third party plugins vendors in "Confluence Admin > Manage Add ons" and check for possible expired licenses.
You will need to follow up with the plugin vendor for renewal.