QuoteThe easiest (temporary) solution is of course to install the latest version 1.x of composer.
Thank you. Adding
--1 to the composer installer installed the latest stable 1.x version of it.composer-setup.php --install-dir=/usr/local/bin --filename=composer --1
The hints on the GitHub release page
QuotePlease use only the files ILIAS-6.6.tar.gz or ILIAS-6.6.zip for download and installing ILIAS and ignore the files offered by GitHub (called 'Source Code') to prevent problems with composer.
were not that helpful in solving the issue for me, as I track everything, including customizations, using Git.