A follow up to this thread: Can't install experimental modules
I've installed Drupal 8.2.5 two times on my server. First with the "minimal installation" option, and after that with the "normal installation" option. In both cases the installation worked without any errors. But in both cases I can't install experimental modules. I set the check mark next to an experimental module, then click install, the site refreshes and nothing changes. I can install other modules just fine. Since I must upgrade an existing Drupal 7 installation, I need the experimental Migrate modules.
No error messages are given, neither in "reports" nor on server level. Does someone know why this pre-alpha-like error occurs and how to avoid it?