Symptoms
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The website displays the following error:
Server Error 500.19 - external server error. There is a problem with the resource you are looking for, and it cannot be displayed.
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Enabled Failed request tracing for example.com shows:
Error: websrvmng failed: Filename: ?C:Inetpubvhostsexample.comhttpdocsweb.config
Line number: 15
Error: There is a duplicate 'system.web.extensions/scripting/scriptResourceHandler' section defined
Cause
The application/website requires a specific version of the framework (ASP.NET 3.5) that is not compatible.
Resolution
Install the required ASP.NET version (ASP.NET 3.5) by following this article: How to enable ASP.NET 3.5 / ASP.NET 4.7 in Plesk for Windows Server