Description of problem:
Engine UI returns "Internal Server Error" and not going back after engine's VM was upgraded from rhevm-appliance-20161116.0-1.el7ev.noarch to ovirt-engine-4.0.6.2-0.1.el7ev.noarch.

Version-Release number of selected component (if applicable):
Components on host:
qemu-kvm-rhev-2.6.0-28.el7_3.1.x86_64
vdsm-4.18.18-1.el7ev.x86_64
ovirt-setup-lib-1.0.2-1.el7ev.noarch
ovirt-imageio-common-0.3.0-0.el7ev.noarch
sanlock-3.4.0-1.el7.x86_64
ovirt-vmconsole-1.0.4-1.el7ev.noarch
mom-0.5.8-1.el7ev.noarch
rhev-release-4.0.6-4-001.noarch
ovirt-hosted-engine-ha-2.0.6-1.el7ev.noarch
ovirt-host-deploy-1.5.3-1.el7ev.noarch
ovirt-engine-sdk-python-3.6.9.1-1.el7ev.noarch
ovirt-imageio-daemon-0.4.0-0.el7ev.noarch
libvirt-client-2.0.0-10.el7.x86_64
ovirt-vmconsole-host-1.0.4-1.el7ev.noarch
rhevm-appliance-20161116.0-1.el7ev.noarch
ovirt-hosted-engine-setup-2.0.4.1-2.el7ev.noarch
Linux version 3.10.0-514.el7.x86_64 (mockbuild.eng.bos.redhat.com) (gcc version 4.8.5 20150623 (Red Hat 4.8.5-11) (GCC) ) #1 SMP Wed Oct 19 11:24:13 EDT 2016
Linux alma04.qa.lab.tlv.redhat.com 3.10.0-514.el7.x86_64 #1 SMP Wed Oct 19 11:24:13 EDT 2016 x86_64 x86_64 x86_64 GNU/Linux
Red Hat Enterprise Linux Server release 7.3 (Maipo)

Components on engine:
rhevm-setup-plugins-4.0.0.3-1.el7ev.noarch
rhevm-spice-client-x86-msi-4.0-3.el7ev.noarch
rhev-release-4.0.6-4-001.noarch
rhevm-dependencies-4.0.0-1.el7ev.noarch
rhev-guest-tools-iso-4.0-6.el7ev.noarch
rhevm-doc-4.0.6-1.el7ev.noarch
rhevm-spice-client-x64-msi-4.0-3.el7ev.noarch
rhevm-4.0.6.2-0.1.el7ev.noarch
rhevm-guest-agent-common-1.0.12-3.el7ev.noarch
rhevm-branding-rhev-4.0.0-6.el7ev.noarch
Linux version 3.10.0-514.2.2.el7.x86_64 (mockbuild.eng.bos.redhat.com) (gcc version 4.8.5 20150623 (Red Hat 4.8.5-11) (GCC) ) #1 SMP Wed Nov 16 13:15:13 EST 2016
Linux 3.10.0-514.2.2.el7.x86_64 #1 SMP Wed Nov 16 13:15:13 EST 2016 x86_64 x86_64 x86_64 GNU/Linux
Red Hat Enterprise Linux Server release 7.3 (Maipo)


How reproducible:
100%

Steps to Reproduce:
1.Deploy HE with rhevm-appliance-20161116.0-1.el7ev.noarch on Conroe host.
2.Once deployed, add some data storage domain to the engine and wait until hosted-storage being imported.
3.Set host in to global maintenance and update repos on appliance to latest bits and yum update the engine.
4.Run engine-setup and finish successfully with it.
5.Remove host from global maintenance.

Actual results:
Engine's VM is up, but WEBUI stuck with "Internal Server Error" on it's web page.

Expected results:
UI should get started without any issues after engine's upgrade.

Additional info:
sosreports added from engine and host.
This issue might be related to https://bugzilla.redhat.com/show_bug.cgi?id=1400619.