I installed a disconnected OCP 4.16 cluster on s390x KVM following the directions here: https://docs.openshift.com/container-platform/4.16/installing/installing_ibm_z/installing-ibm-z-kvm.html. Once the cluster was up, I followed these instructions (https://docs.openshift.com/container-platform/4.16/updating/updating_a_cluster/migrating-to-multi-payload.html#migrating-to-multi-payload) to prepare the cluster for adding x86 worker nodes. Using the provided "oc adm upgrade --to-multi-arch" command fails. 

    4.16.3

Possible to reproduce    

    1. Install disconnected s390x cluster from media mirrored to a local registry
    2. Run "oc adm upgrade --to-multi-arch"
    3. 
    

    The upgrade spins and running oc adm upgrade returns:
"Cluster version is 4.16.3 Upstream is unset, so the cluster will use an appropriate default.Channel: stable-4.16warning: Cannot display available updates:  Reason: RemoteFailed  Message: Unable to retrieve available updates: Get "https://api.openshift.com/api/upgrades_info/v1/graph?arch=multi&channel=stable-4.16&id=3cf61391-f0a1-4ce9-b19b-d7c9669f9c99&version=4.16.3": dial tcp 54.147.93.38:443: connect: connection timed out"

    The upgrade proceeds as expected

With the "multi" channel media mirrored to the local registry, the upgrade to multi-arch succeeded with the following command   
"oc adm upgrade --force --allow-upgrade-with-warnings --allow-explicit-upgrade --to-image <local_registry>/<my_cluster>/openshift4/16/install/openshift/release-images:4.16.3-multi"