How to fix the joomla error "Error displaying the error"

Joomla will show the "Error displaying the error page: Application Instantiation Error" if your configuration.php file is not correctly configured. I.E. if the information used to connect to your Joopmla databes is NOT correct, you will always get the "Application Instantiation Error"

So just check again and again that database user and password is correct and mysql or mysqli extension is used.