subdirectory – Drupal 7 in the root directory, Drupal 8 in the subdirectory – Unable to access Drupal 8

In a strange situation, a site running Drupal 7 is installed in the root directory. I want to install a Drupal 8 site in a subdirectory – but when I go to, I'm redirected to the Drupal 7 page not found. It should be noted that there is another installation of Drupal 7 in another subdirectory and that this site is accessible via I imagine that this may have something to do with .htaccess but that we have failed so far.

Most of what I've been able to find discusses subdirectory installations where they want to hide the path to the OU subdirectory address issues related to multisite installations.