CI 3.1.13 Unable to locate the specified class: Session.php

So I've inherited an old CI 3 project running on PHP7.0

I've upgrade to the latest CI 3 version and I want to make the change to PHP7.4 so that I can keep upgrading to CI 4, then upgrade to PHP8 etc.

95% of the site works fine but on some pages, I just get a blank page saying "Unable to locate the specified class: Session.php"


These are my autoloaded libraries:

Code:

$autoload['libraries'] = array('database','session','lib_smarty','module','form','tank_auth','user_auth');

I've looked through the forum for topics with a similar problem but most were either unresolved and/or their solution didn't work for me.
I've already installed a fresh copy of CI 3.1.13.

If I remove all libraries from the autoload, I am greeted with the following error.

Code:

A PHP Error was encountered

Severity: Notice

Message: Undefined property: Module::$session

Filename: core/Model.php

Line Number: 74