I ran the upgrade from the web console, on 4 identical systems, same hardware, yet only one has this issue... very puzzling...
I am able to login via SSH without issue, only the webui console is missing the realm.
root@pve-2:~# pveversion -v
proxmox-ve: 5.1-42 (running kernel: 4.13.16-2-pve)
pve-manager: 5.1-51 (running version: 5.1-51/96be5354)
pve-kernel-4.13: 5.1-44
pve-kernel-4.13.16-2-pve: 4.13.16-47
pve-kernel-4.13.16-1-pve: 4.13.16-46
pve-kernel-4.13.4-1-pve: 4.13.4-26
pve-kernel-4.10.15-1-pve: 4.10.15-15
corosync: 2.4.2-pve4
criu: 2.11.1-1~bpo90
glusterfs-client: 3.8.8-1
ksm-control-daemon: 1.2-2
libjs-extjs: 6.0.1-2
libpve-access-control: 5.0-8
libpve-apiclient-perl: 2.0-4
libpve-common-perl: 5.0-30
libpve-guest-common-perl: 2.0-14
libpve-http-server-perl: 2.0-8
libpve-storage-perl: 5.0-18
libqb0: 1.0.1-1
lvm2: 2.02.168-pve6
lxc-pve: 3.0.0-2
lxcfs: 3.0.0-1
novnc-pve: 0.6-4
proxmox-widget-toolkit: 1.0-15
pve-cluster: 5.0-25
pve-container: 2.0-21
pve-docs: 5.1-17
pve-firewall: 3.0-8
pve-firmware: 2.0-4
pve-ha-manager: 2.0-5
pve-i18n: 1.0-4
pve-libspice-server1: 0.12.8-3
pve-qemu-kvm: 2.11.1-5
pve-xtermjs: 1.0-2
qemu-server: 5.0-25
smartmontools: 6.5+svn4324-1
spiceterm: 3.0-5
vncterm: 1.5-3
zfsutils-linux: 0.7.7-pve1~bpo9
root@pve-2:~# uname -a
Linux pve-2.local.ben.io 4.13.16-2-pve #1 SMP PVE 4.13.16-47 (Mon, 9 Apr 2018 09:58:12 +0200) x86_64 GNU/Linux