@amateescu Just confirming - did you executed these two commands php modules/...

@amateescu Just confirming - did you executed these two commands php modules/composer_manager/scripts/init.php, composer drupal-update after downloading composer_manager. And composer_manager does not needs to be installed.

Weird that you are not able to reproduce the problem :/

Apart from the steps you don't have to install any other modules, not even commerce module. I will try to setup a simplytest.me instance.