If you have iOS devices connected to your IBM Sametime 8.5.2.x environment you will need to update the security certificates on the ST proxy server before end of September: IBM Updated security certificate for Push Notifications (iOS)
Websphere: Replacing the default IBM self-signed certificate with another certificate
Just in case I will need it the next time … This article is a good description how to Adding trusted root and intermediary certificates to the Cell default trust store Generating a certificate signer request Installing a signed certificate