twig dump() creates WSOD error
I've solved this.
In Drupal, modules also use composer (which I wasn't aware of). The vardumper module depends on the symfony/var-dumper package.
Running:
composer require --dev symfony/var-dumper
causes dependency conflicts, however using:
composer update
Will install the var-dumper package, fix the problem - now I can see very beautiful variable dumps.
Lesson learnt:

Always check composer for dependency errors.