Problem
After upgrading Exchange to CU23 ECP and OWA no longer work. After logging in to the splash screen Owa was throwing a 500 error. ECP was showing “Could not load file or assembly ‘Microsoft.Exchange.Common, Version=15.0.0.0 …”
Resolution
Open IIS Manager -> Expand Sites-> Exchange Back End
Click ECP. Open the Application Settings in the middle column
Please check whether the value for “BinSearchFolders” is changed to an invalid value or %ExchangeInstall% or %ExchangeInstallPath%
For example, if your exchange file installation is on default location, 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
Run IISRESET from command prompt
If the ECP is still showing an error. 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\
Check the web.config file for ECP in Exchange back end and confirm 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
If after all steps above the ECP still showing an error, run UpdateCas.PS1 under “bin” folder on the exchange directory installation