When I activate the theme in a clean WordPress installation and then enter the demos and select the minimalist I always have a server error 500 message.
Then I can’t even visit the WordPress control panel.
500 Internal Error is a generic error and says about some issue on the server and can be caused by different things. Anyway, to understand its nature you need to see your server error logs or enable PHP error display on the website. If you can’t do this, you need to contact your hosting provider for help in this question. Here is an article that may help you understand the error better http://www.wpbeginner.com/wp-tutorials/how-to-fix-the-internal-server-error-in-wordpress/