If you want to use cmi5/xAPI/Tin Can with ILIAS you have to activate HTTP Authorization Header.
Please ensure that in your Apache configuration are the following lines in the section 'FilesMatch \.php$':

SetEnvIf Authorization "(.*)" HTTP_AUTHORIZATION=$1

You might to have activate SetEnvIf with:

a2enmod setenvif

Don't forget to restart Apache e.g. with:

systemctl restart apache2