When applying Exchange Server 2013 CU23 to hosts in a three node DAG (all Multi-Role), I hit an issue where I couldn’t launch the CU GUI installer on one of the hosts.
CU23 installed successfully on the other two hosts.
The following error (Event ID 4999) was logged.
Log Name: Application
Source: MSExchange Common
Event ID: 4999
Watson report about to be sent for process id: 11732, with parameters: E12IIS, c-RTL-AMD64, 15.00.1497.000, ExSetupUI, ExSetupUI, M.E.S.E.SetupWizard.PopulateWizard, M.E.S.ExSetupUI.AssemblyLoadFileNotFoundException, a60d, 15.00.1497.000.
From an elevated Command Prompt, CD to the folder containing the extracted CU, and run the following command:
setup /M:Upgrade /IAcceptExchangeServerLicenseTerms