RHOCP upgrade from v4.5.16 to v4.6.8 is stuck on 20% for several hours

Issue

• RHOCP upgrade is not making any progress for past few hours
• clusterversion does not show any error. None of the cluster operators are in Progressing or Degraded state but they are on mixed versions ( v4.5.16 and v4.6.8 )

$ oc get clusterversion
NAME      VERSION   AVAILABLE   PROGRESSING   SINCE   STATUS
version   4.5.16    True        True          5h20m   Working towards 4.6.8: 20% complete

$ oc get co 
NAME                                       VERSION   AVAILABLE   PROGRESSING   DEGRADED   SINCE
authentication                             4.5.16    True        False         False      15h
cloud-credential                           4.5.16    True        False         False      261d
cluster-autoscaler                         4.5.16    True        False         False      261d
config-operator                            4.5.16    True        False         False      30d
console                                    4.5.16    True        False         False      30d
csi-snapshot-controller                    4.5.16    True        False         False      20d
dns                                        4.5.16    True        False         False      261d
etcd                                       4.6.8     True        False         False      30d
image-registry                             4.5.16    True        False         False      30d
ingress                                    4.5.16    True        False         False      5h2m
insights                                   4.5.16    True        False         False      261d
kube-apiserver                             4.6.8     True        False         False      261d
kube-controller-manager                    4.5.16    True        False         False      30d
kube-scheduler                             4.6.8     True        False         False      30d
kube-storage-version-migrator              4.5.16    True        False         False      30d
machine-api                                4.5.16    True        False         False      261d
machine-approver                           4.5.16    True        False         False      30d
machine-config                             4.5.16    True        False         False      5h18m
marketplace                                4.5.16    True        False         False      5h2m
monitoring                                 4.5.16    True        False         False      12m
network                                    4.5.16    True        False         False      261d
node-tuning                                4.5.16    True        False         False      30d
openshift-apiserver                        4.5.16    True        False         False      13d
openshift-controller-manager               4.5.16    True        False         False      20d
openshift-samples                          4.5.16    True        False         False      30d
operator-lifecycle-manager                 4.5.16    True        False         False      261d
operator-lifecycle-manager-catalog         4.5.16    True        False         False      261d
operator-lifecycle-manager-packageserver   4.5.16    True        False         False      3d18h
service-ca                                 4.5.16    True        False         False      261d
storage                                    4.5.16    True        False         False      20d