. Security Update For Exchange Server 2013 CU23 (KB5000871) won't be installed.
Application event logs shows:
Product: Microsoft Exchange Server - Update 'Security Update for Exchange Server 2013 Cumulative Update 23 (KB5000871) 15.0.1497.12' could not be installed. Error code 1603. Windows Installer can create logs to help troubleshoot issues with installing software packages. Use the following link for instructions on turning on logging support:
CU23 was installed successfully