Okay so I changed 'twig' in the always_load array back to 'parser', and if I am right, now I just have to changes the extensions of all .php files in the views folder to .twig?
You don't have to, if you want to use twig as the parser for all your templates, and you will never use standard templates anymore, you can also configure the parser to map the .php extension to twig. But that means you also have to create twig templates for all views in fuel/core.