After applying CU21 on Exchange 2013, I am unable to access ECP. Error in event log:
Event code: 3008
Event message: A configuration error has occurred. 
Event ID: b03040db26554e5aab7f6976d7878148
Event sequence: 1
Event occurrence: 1
Event detail code: 0
Application information:     
Application domain: /LM/W3SVC/2/ROOT/ecp-17-131950660982668178    
Trust level: Full
Application Virtual Path: /ecp
Application Path: C:\Program Files\Microsoft\Exchange Server\V1\ClientAccess\ecp\

Exception information: Exception type: ConfigurationErrorsException Exception message: Could not load file or assembly ‘Microsoft.Exchange.Common, Version=15.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35’ or one of its dependencies. The system cannot find the file specified.

Solution:

1. Login to your Exchange server.
2. Launch Exchange Management Shell or PowerShell.
3. Navigate to your Exchange 2013 binaries location, C:\Program Files\Microsoft Server\V15\Bin\
4. Execute the UpdateCas.ps1 Windows PowerShell Script and wait to complete.
5. Also execute UpdateConfigFiles.ps1.