UEFI-mode Bionic deployments failing

Ubuntu 18.04 deployments in UEFI mode have recently begun failing at GRUB installation. The attempt produces normal installation output up until the GRUB installation, when this appears:

Setting up shim (13-0ubuntu2) ...
Setting up libefivar1:amd64 (34-1) ...
Processing triggers for libc-bin (2.27-3ubuntu1) ...
Setting up sbsigntool (0.6-3.2ubuntu2) ...
Setting up libefiboot1:amd64 (34-1) ...
Processing triggers for man-db (2.8.3-2) ...
Setting up mokutil (0.3.0-0ubuntu5) ...
Setting up secureboot-db (1.1) ...
Can't access efivars filesystem at /sys/firmware/efi/efivars, aborting
Setting up efibootmgr (15-1) ...
Setting up grub-efi-amd64-bin (2.02-2ubuntu8) ...
Setting up grub-efi-amd64 (2.02-2ubuntu8) ...
Replacing config file /etc/default/grub with new version
Setting up shim-signed (1.34.6+13-0ubuntu2) ...
Setting up grub-efi-amd64-signed (1.93+2.02-2ubuntu8) ...
Processing triggers for libc-bin (2.27-3ubuntu1) ...
Unexpected error while running command.
Command: ['sh', '-c', 'exec "$0" "$@" 2>&1', 'install-grub', '--uefi', '--update-nvram', '/tmp/tmpuek34532/target', '/dev/sda']
Exit code: 1
Reason: -
Stdout: + echo before grub-install efiboot settings
        before grub-install efiboot settings
        + efibootmgr
        BootCurrent: 0002
        Timeout: 1 seconds
        BootOrder: 0002,0003,0001
        Boot0001* UEFI C400-MTFDDAT064MAM MSA1727018J
        Boot0002* UEFI PXEv4 (MAC:ECA86BFF33AD)
        Boot0003* UEFI PXEv6 (MAC:ECA86BFF33AD)
        + dpkg-reconfigure grub-efi-amd64
        Replacing config file /etc/default/grub with new version
        Processing triggers for shim-signed (1.34.6+13-0ubuntu2) ...
        No MOK found.
        dpkg: error processing package shim-signed (--configure):
         installed shim-signed package post-installation script subprocess returned error exit status 1
        Errors were encountered while processing:
         shim-signed
        failed to install grub!

Stderr: ''
curtin: Installation failed with exception: Unexpected error while running command.
Command: ['curtin', 'curthooks']
Exit code: 3
Reason: -

The first sign of trouble that I see is the complaint about being unable to access /sys/firmware/efi/efivars.

I've reproduced this problem occurs on about half a dozen nodes on three MAAS servers. The problem does NOT occur when deploying Ubuntu 16.04, but DOES occur when deploying the latest pre-release 18.04. I'm attaching a tarball of /var/log/maas from one of the MAAS servers. The test system on that server is brennan.

$ dpkg -l '*maas*'|cat
Desired=Unknown/Install/Remove/Purge/Hold
| Status=Not/Inst/Conf-files/Unpacked/halF-conf/Half-inst/trig-aWait/Trig-pend
|/ Err?=(none)/Reinst-required (Status,Err: uppercase=bad)
||/ Name Version Architecture Description
+++-===============================-====================================-============-==================================================
ii maas 2.3.2-6485-ge93e044-0ubuntu1~16.04.1 all "Metal as a Service" is a physical cloud and IPAM
ii maas-cert-server 0.2.36-0~201804111925~ubuntu16.04.1 all Ubuntu certification support files for MAAS server
ii maas-cli 2.3.2-6485-ge93e044-0ubuntu1~16.04.1 all MAAS client and command-line interface
un maas-cluster-controller <none> <none> (no description available)
ii maas-common 2.3.2-6485-ge93e044-0ubuntu1~16.04.1 all MAAS server common files
ii maas-dhcp 2.3.2-6485-ge93e044-0ubuntu1~16.04.1 all MAAS DHCP server
ii maas-dns 2.3.2-6485-ge93e044-0ubuntu1~16.04.1 all MAAS DNS server
ii maas-proxy 2.3.2-6485-ge93e044-0ubuntu1~16.04.1 all MAAS Caching Proxy
ii maas-rack-controller 2.3.2-6485-ge93e044-0ubuntu1~16.04.1 all Rack Controller for MAAS
ii maas-region-api 2.3.2-6485-ge93e044-0ubuntu1~16.04.1 all Region controller API service for MAAS
ii maas-region-controller 2.3.2-6485-ge93e044-0ubuntu1~16.04.1 all Region Controller for MAAS
un maas-region-controller-min <none> <none> (no description available)
un python-django-maas <none> <none> (no description available)
un python-maas-client <none> <none> (no description available)
un python-maas-provisioningserver <none> <none> (no description available)
ii python3-django-maas 2.3.2-6485-ge93e044-0ubuntu1~16.04.1 all MAAS server Django web framework (Python 3)
ii python3-maas-client 2.3.2-6485-ge93e044-0ubuntu1~16.04.1 all MAAS python API client (Python 3)
ii python3-maas-provisioningserver 2.3.2-6485-ge93e044-0ubuntu1~16.04.1 all MAAS server provisioning libraries (Python 3)