Microsoft Exchange security updates may have installation issues on servers where User Account Control (UAC) is enabled. The issue occurs because some files are not updated when installing the security updates manually by double-clicking the MSP installers as a normal user. Microsoft recommends installing the updates as an administrator from the command-line. The only indication that something might be wrong is that Outlook on the web and the Exchange Control Panel (ECP) might suddenly stop working. Researchers and Microsoft employees have also been tweeting warnings to users to make sure the patches are being deployed on these problematic servers.
Source: https://www.bleepingcomputer.com/news/security/microsoft-exchange-updates-can-install-without-fixing-vulnerabilities/