Website is not accessible: 500 Internal Server Error: Config section already defined

Symptoms

• Website shows:

  PLESK_INFO: 500 Internal Server Error

• An attempt to reconfigure the website fails:

  C:\> plesk repair web example.com
  Repairing web server configuration
  Repairing web server configuration for the domain example.com ..... [FAILED]
  - Filename: \\?\D:\Inetpub\vhosts\example.com\httpdocs\web.config
  Line number: 8
  Error: Config section 'system.web/compilation' already defined.
  Sections must only appear once per config file. See the help
  topic for exceptions

• Accessing IIS > Sites > example.com > ASP results in error message:
  

Cause

Duplicate section in domain configuration file.

Resolution

1. Login to the server over RDP

2. Remove or comment out the duplicate section mentioned in the error message from %plesk_vhosts%/example.com/httpdocs/web.config:

   CONFIG_TEXT: <compilation debug="true" targetFramework="4.0">
   <assemblies>
   <add assembly="Microsoft.Build.Framework, Version=4.0.0.0, Culture=neutral, PublicKeyToken=************" />
   <add assembly="System.Management, Version=4.0.0.0, Culture=neutral, PublicKeyToken=************" />
   </assemblies>
   <buildProviders>
   <add extension=".rdlc" type="Microsoft.Reporting.RdlBuildProvider, Microsoft.ReportViewer.WebForms, Version=10.0.0.0, Culture=neutral, PublicKeyToken=***********" />
   </buildProviders>
   </compilation>

3. Reconfigure the domain:

   C:\> plesk repair web example.com

The following article may be helpful in this case: http://www.asp.net/whitepapers/aspnet4/breaking-changes#0.1__Toc256770149