Installation on local xampp

Problem/Motivation

Steps to reproduce

• Installing on local Xampp
• created new root folder
• used the default composer-line to install non-dev version:
  composer create-project Vardot/varbase-project:~9 avvar --no-dev --no-interaction
• After a few moments, the installation is finished (although patch 2869592 could not be applied), therefore I go to "localhost/avvar/docroot/core/install.php"
• Then it goes wrong...

The error I get in the browser:

Warning: require_once(C:\xampp\htdocs\avvar\docroot\autoload.php): failed to open stream: No such file or directory in C:\xampp\htdocs\avvar\docroot\core\install.php on line 38

Fatal error: require_once(): Failed opening required 'C:\xampp\htdocs\avvar\docroot\autoload.php' (include_path='C:\xampp\php\PEAR') in C:\xampp\htdocs\avvar\docroot\core\install.php on line 38

When I look in the docroot-folder, there is no file at all...(only the folders with all content).

What additional information is needed to investigate this issue?

PS: I've tried to install a clean drupal installation using composer as well, which succeeded without any problems.