Unable to access ASP.NET Settings of a domain in Plesk: Can’t detect .NET version. Possible site does not use ASP.NET

Symptoms

• The following error message is shown when:

  • opening ASP.NET settings in Plesk at Domains > example.com > ASP.NET Settings.

  • changing password of the main FTP user at Domains > example.com > FTP Access.

  • issuing a Let's Encrypt certificate for a domain.

  aspnetcfg failed: Can't detect .NET version. Possible site does not use ASP.NET (Error code 1)

• The website may also load with the error message:

  The page cannot be displayed because an external server error has occurred.

Cause

Misconfiguration in IIS:

• .NET CLR version is set to No Managed Code in application pool settings of the website in IIS Manager > HOSTNAME > Application Pools > example.com.
• Microsoft ASP.NET support is enabled in Plesk at Domains > example.com > Hosting & DNS > Hosting Settings.

Resolution

1. Log in to Plesk.

2. Go to Domains > example.com > Hosting & DNS > Hosting Settings.

3. Disable the option Microsoft ASP.NET support and click Apply.

4. Enable the option Microsoft ASP.NET support back and click Apply.