> All of the packages marked as Download-release.zip … come up as not found.

Thanks for the info, maybe I did an error once and copy and pasted it again and again. The .zip at the bottom of each release in Assests should work.

> Both display an internal server error when you point the browser to the folder.

The php_intl extension is missing in the PHP installation. See also the first answer: https://stackoverflow.com/a/26319843

Detailed requirements for PHP-extensions of the underlying framework can be found on the top of the page here: https://book.cakephp.org/3/en/installation.html

Forum itself requires the gd and exif extension.

Summing it up you should install gd, exif, intl, mbstring, simplexml and pdo_mysql.