Plesk site on shared applicationPool shows error 500: The FastCGI process exited unexpectedly

Symptoms

• The website example.com is unavailable with the error message:

  HTTP Error 500.0 - external Server Error
  C:Program Files (x86)PleskAdditionalPleskPHP70php-cgi.exe - The FastCGI process exited unexpectedly

• More than one site is using shared application pool "plesk(default)(4.0)(pool)" at Windows > Server Manager > Tools > IIS Manager > hostname > Application Pools > plesk(default)(4.0)(pool) > View Applications. 

Cause

Domains with different ASP.NET versions use shared application pool.

Resolution

1. Log into Plesk.
2. Enable dedicated application pool for both websites in Domains > example.com > Dedicated IIS Application Pool for Website > Switch On.
3. Disable it again in Domains > example.com > Dedicated IIS Application Pool for Website > Switch Off.