Apple issues certificates to developers who distribute software outside the App Store, like Symantec. These certificates expire over time and need to be renewed through Apple. It is possible to click "Continue" to install the client package with the expired certificate anyway, although updating to a newer version is recommended.
Symantec updated the certificate used in the Encryption Desktop client for MacOS in client version 10.4.2 MP1, which is included in the Encryption Management Server 3.4.2 MP1 release. Download the updated packages from Symantec at http://fileconnect.symantec.com/ or through your distribution sources.