This bug exists since Fedora 22 (several Bugzilla reports have been filed for this issue but none has been processed yet).
The Server image does not have this problem and it can be used as a workaround to setup a minimal Server, and then install the "Workstation" package group from network remotely (using dnf).
But if there definitely appears to be no activity after several hours, shut the system down.
Now either boot up and add There was an unfortunate situation in the past few Fedora releases where Package Kit and DNF didn't work well together.
It is also possible that this might happen to you when performing a system upgrade to Fedora 24. Future use of Package Kit (GNOME Software, Apper) should be safe.
However, if you have ever used these tools in the past, you're strongly advised to fix your "user installed" database before you start using DNF: Please note that this solution is slightly excessive, because you're going to end up with all your packages considered either system essential or user requested, and none of them is ever going to be removed as a no-longer-needed dependency.
Power users can tweak this solution according to their needs.The result is not exactly the same and requires some minor additional manual work afterwards.(If anybody knows easier workarounds, please add to this section.) This is a third-party repository issue outside of Fedora control.You can read more about possible solutions in this blog post.The repository maintainers are supposedly working on signing their repo, so that the issue disappears in the future.