ecp 접속 시, 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.
Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.

1. Open IIS Manager. Expand Sites > Exchange Back End.
2. Click ecp. Open Application Settings in /ecp Home.
3. Please check whether the value for “BinSearchFolders” is changed to an invalid value. If so, please change it to:
C:\Program Files\Microsoft\Exchange Server\V15\bin;C:\Program Files\Microsoft\Exchange Server\V15\bin\CmdletExtensionAgents;C:\Program Files\Microsoft\Exchange Server\V15\ClientAccess\Owa\bin
4. Open Web.config file for OWA in the default path C:\Program Files\Microsoft\Exchange Server\V15\ClientAccess\Owa. If there is any %ExchangeInstallDir% in the web.config content, replace all of them with C:\Program Files\Microsoft\Exchange Server\V15\.
5. Please check the web.config file for ECP in Exchange back end and confim if there is any %ExchangeInstallDir% in the web.config content. If that is the case, please replace all of %ExchangeInstallPath% to c:\programfiles\Microsoft\Exchange Server\V15\Bin.
6. Run IISReset command to restart IIS services.

If all fails, please ran the command updateCas.PS1 then check whether the issue persists.
(C:\Program Files\Microsoft\Exchange Server\V15\Bin and then execute the power shell script UpdateCas.ps1)