The solution was a combination of multiple things:
the boot-repair did work in that it successfully reinstalled grub.
however, it was also necessary to set the newly installed grub version as trusted in UEFI, as pointed out by oldfred above (thank you very much!); refer to Bootable device not found after clean install of Ubuntu 14.04 UEFI on Acer for more ...
During grub-install needed files are copied from /usr/lib/grub/i386-pc/ to /boot/grub/i386-pc/
Maybe this works. Can you boot a live-linux? Mount your /-Partition from your system and try to copy the missing file and reboot. When you have access to your System reinstall grub and update grub-configuration.
The issue was caused by a plugged-in USB headset. Because I was unable to perform any of your suggestions, I felt it had to be either a damaged disk, or something incredibly stupid.
After a successful disk check, I unplugged all USB devices from the PC as a last measure before doing a reload, and it booted-up perfectly.
Assuming you are using Ubuntu 15.04 or newer, you can disable individual services by running this command in a terminal:
sudo systemctl disable service-to-disable
For example, if you wanted to disable snapd:
sudo systemctl disable snapd
(Do not include the .service extension when referring to a service on systemctl.)
Be careful when disabling a service, ...
A good workaround for now is to install the rescue software Super Grub Disk 2 on a small USB stick that sits permanent in the USB slot of the computer. There is a software YUMI – Your Universal Multiboot Installer that can install the super_grub2_disk_hybrid_2.02s10.iso on a USB stick. Ubuntu boots from the rescue USB stick, decrypts the LVM volume and works ...
I found some interesting stuff the net. Super Grub Disk 2 is a software, that makes it possible to boot totally messed up Linux systems that use GRUB2. This works also with encrypted volumes (Cryptosetup-Luks or Logical Volume Manager LVM). After downloading and burning the ISO-file, I started the computer from this CD. Ubuntu booted without any problems, so ...
Scott Severance's answer is valid and detailed but there is a resolution that requires no external boot device and so no need to identify and manually mount all of your Ubuntu partitions leading up to the chroot.
In Windows 10, you can use Advanced Recovery to select a device (partition?) to boot from.
Go to Settings and choose Update & Security: