I just tried vagrant (using libvirt) and it works, but gnome-boxes does not.

I don't know. But only suggested solution, adding kvm group did change the error messages, but there is still some problem.

Also the log contains things like this:

Adding kvm to my user did solve something. Then I also installed libvirt-daemon-system manually, because it is only suggested.
But still I get some error when creating vm: