Website loads without CSS or images are not loaded: Cannot add duplicate collection entry of type 'mimeMap'

Symptoms

• CSS is not working for a website or some images (.webp) are not loaded with the following error in a browser console:

  CONFIG_TEXT: net::ERR_ABORTED 500 (Internal Server Error)

• HTML pages are not loaded with this error:

  CONFIG_TEXT: The page cannot be displayed because an internal server error has occurred.

• Failed Request Tracing in Domains > example.com shows the following error:

  PLESK_ERROR: ModuleName CustomErrorModule
  Notification SEND_RESPONSE
  HttpStatus 500
  HttpReason Internal Server Error
  HttpSubStatus 19
  ErrorCode Cannot create a file when that file already exists.
  (0x800700b7)
  ConfigExceptionInfo \?\C:\Inetpub\vhosts\example.com\httpdocs\web.config ( 45) :Cannot add duplicate collection entry of type 'mimeMap' with unique key attribute 'fileExtension' set to '.webp'

Cause

Incorrect "staticContent" section in the web.config file: it is duplicated on IIS level and in domain's configuration file.

Resolution

1. Log in to Plesk.

2. Navigate to Domains > example.com > File manager and open the file web.config.

3. Add the <clear /> before the line number reported in the error message:

   CONFIG_TEXT: 43 <staticContent>
   44 <clientCache cacheControlCustom="public" cacheControlMode="UseMaxAge" cacheControlMaxAge="365.00:00:00" />
   45 <clear /> <!-- Add this -->
   46 <mimeMap fileExtension=".webp" mimeType="image/webp" />
   47 </staticContent>

Note: When many domains are affected, apply the same solution for %windir%\system32\inetsrv\config\applicationHost.config file or remove the line reported in the error from it.