One upgrade to F23 succeeded. The second server fails, repeatedly with
"dnf system-upgrade --releasever 23 download" resulting in:
…
[SKIPPED] zukitwo-gtk3-theme-0.0.2-1.fc23.noarch.rpm: Already
downloaded
[SKIPPED] zukitwo-gtk2-theme-0.0.2-1.fc23.noarch.rpm: Already
downloaded
warning: /var/lib/dnf/system-upgrade/kernel-core-4.2.5-300.fc23.x86_64.rpm:
Header V3 RSA/SHA256 Signature, key ID 34ec9cba: NOKEY
Curl error (37): Couldn't read a file:// file for file:///etc/pki/rpm-
gpg/RPM-GPG-KEY-fedora-x86_64 [Couldn't open file /etc/pki/rpm-gpg/RPM-GPG-
KEY-fedora-x86_64]
The downloaded packages were saved in cache till the next successful
transaction.
You can remove cached packages by executing 'dnf clean packages'.
Googling around doesn't find anything useful.