Today I was updating Exchange Server 2016 CU 18 on one of the client’s Exchange Servers and I came across the below error. The error is the original web.config file was not able to back up because of a web.config.bak file was already there in the %ExchangeInstall%\ClientAccess\ Autodiscover\. Error: ” was run: “System.UnauthorizedAccessException”

Error:

" was run: "System.UnauthorizedAccessException: Access to the path 'C:\Program Files\Microsoft\Exchange Server\V15\ClientAccess\Autodiscover\web.config.bak' is denied. at 
System.IO
.__Error.WinIOError(Int32 errorCode, String maybeFullPath) at 
System.IO
.File.InternalCopy(String sourceFileName, String destFileName, Boolean overwrite, Boolean checkHost) at 
Microsoft.Exchange.Management
.SetInstallPathInAppConfig.InternalProcessRecord() at 
Microsoft.Exchange
.Configuration.Tasks.Task.<ProcessRecord>b__91_1() at 
Microsoft.Exchange
.Configuration.Tasks.Task.InvokeRetryableFunc(String funcName, Action func, Boolean terminatePipelineIfFailed) at 
Microsoft.Exchange
.Configuration.Tasks.Task.ProcessTaskStage(TaskStage taskStage, Action initFunc, Action mainFunc, Action completeFunc) at 
Microsoft.Exchange
.Configuration.Tasks.Task.ProcessRecord() at 
System.Management
.Automation.CommandProcessor.ProcessRecord()".

Solution:

In order to resolve this issue, I have backed up the existing web.config.bak file and removed this file from the location above. I reran the setup again and, Bingo! There was no error on the stage and the Cumulative update was completed with charm. I recommend this solution if you encounter this issue in your Exchange Server 2013/1016/2019 cumulative update upgrade.

Please feel to comment your thoughts and I will be right there to answer you in case of any questions.