Hi,
So what procedures did you try exactly ?
Did you try changing the line $dbtype = 'mysqli'; to $dbtype = 'mysql'; in the configuration.php file of Joomla ?
Did you empty the session table ? Did you try repairing it ?
On these links, someone also recommends trying this:
after trying all the fixes you listed Andrew, and countless others from the previous comments and StackOverflow, Providence finally smiled on me. It seems all the folders within the libraries/fof folder were doubled. My issue started with updating Akeeba, then the errors I got showed it was a partial install so I had to manually copy some files to the server. These files include the fof table content. By deleting everything and uploading anew, the 'application instantiation' error was sorted.
did you try ?
Someone else also recommends increasing the memory limit of the server.
Also, check the PHP error log of your server, maybe there is a fatal error corresponding to when you get the error which would help understand why this happens.