Fedora 28 was released a while ago, and I do not immediately upgrade my workstation from past experience with external repositories needing to catch up.

Starting with

sudo dnf --refresh upgrade
sudo dnf system-upgrade download --refresh --releasever=28

led to this error.

Fehler: 
 Problem: nss-pem-1.0.3-6.fc27.i686 has inferior architecture
  - nss-pem-1.0.3-6.fc27.x86_64 does not belong to a distupgrade repository
  - problem with installed package nss-pem-1.0.3-6.fc27.i686

This is coming from steam which is installed via UnitedRPM/RPMFusion repository. There seems to be a package dependency change during the F27 cycle, where right now there’s no explicit dependency or provider is there.

sudo dnf remove nss-pem-1.0.3-6.fc27.i686
Abhängigkeiten sind aufgelöst.
=========================================================================================================================================================================
 Paket                                     Arch                        Version                                   Paketquelle                                       Größe
=========================================================================================================================================================================
Entfernen:
 nss-pem                                   i686                        1.0.3-6.fc27                              @updates                                          215 k
Removing dependent packages:
 libcurl                                   i686                        7.55.1-10.fc27                            @updates                                          581 k
 libdbusmenu-gtk2                          i686                        16.04.0-4.fc27                            @fedora                                            79 k
 libdbusmenu-gtk3                          i686                        16.04.0-4.fc27                            @fedora                                            79 k
 nss                                       i686                        3.36.1-1.0.fc27                           @updates                                          2.4 M
 openldap                                  i686                        2.4.45-4.fc27                             @updates                                          1.0 M
 steam                                     i686                        1.0.0.54-17.fc27                          @rpmfusion-nonfree-updates                        2.7 M

This bug report and this blog post helped a lot, just pre-install the F28 package prior to the upgrade. In my case I had to force the package update.

sudo dnf update nss-pem --releasever=28 --best --allowerasing

Next, proceed with the upgrade again.

sudo dnf system-upgrade download --refresh --releasever=28

Say hi to Fedora 28 🙂