Unexpected class in ControllerResolver.php on line 151
My localhost is running PHP 5.5.9. I hope this is not the problem because there won't be anything I can do about it.


Well there is something you can do - change your hosting provider :)

The error is indeed caused by your version of PHP - it's too old. It doesn't understand the newer method of class name resolution, introduced in 5.5. A quick search through core reveals 350+ uses, and I think there is/was an issue around somewhere to convert any uses of static strings as class names to the new way, so it's not something you could realistically work around with patches.

The PHP requirements do list 5.5.9 as a minimum version (until Drupal 8.7.0), so trying to install on anything lower than that is doomed to failure unfortunately.