ErrorException: ini_set() A session is active.

What is your header helper ?
You must not call PHP header() function in CI4 app.
All headers are managed by CI4 Response object.

My header helper literally just loads a few pure php files.

header.php
--> starts php session, so it can check whether or not user is logged in; redirecting user if not logged in
--> loads some javascript functions
--> displays navigation menu's that are the same for all pages

common_functions.php
--> just a bunch of php functions that are used throughout the site