see the issue https://jira.ez.no/browse/EZP-24552 for error messages (same)

I downloaded the released on share.ez.no : http://share.ez.no/downloads/downloads/ez-platform-15.05-alpha and followed the instructions in the INSTALL.md file in the compressed file I got.

The file INSTALL.md included in the tar.gz file available on share.ez.no for the Community 2015.05 version is containing instructions to use demo-clean. After this step, the instance of eZ Platform is showing a 404 page on / and a 500 error page on others URLS (like /login)

This instruction leads to a non-working installation of the Community 2015.05 :

php ezpublish/console ezplatform:install --env prod demo-clean

Either change the instruction to demo, or clean in the tar.gz release file
or fix the demo-clean data