I had a similar problem with only a temp account available and no way to log into my main logon account, as I have a full image backup, I decided to see if I could solve my issues.
1: 1st time I managed to roll back the update., everything appeared to be back to normal. Ran the usual Windows repair tools and they found no problems with the window installation, files or system.
2: Tried to re-install the update and same condition, however a second user account I had created previously worked.
3: Once again Recovered the partition from my backup (about 6 minutes)
4: Downloaded the update directly to the drive and tried installing it.
5: Same problem, could not log into main account, only a temp account showed up with default almost blank desktop and an error message about logging out and back in to fix the problem etc.
At this point I tried everything suggested on the net, editing the reg, powering on / off etc, all made no difference, in the end the admin account went south followed by the user account.
My fix:
1: I again recovered the partition from my backup - so back to square 1, ran the usual Windows repair tools again and they found no problems with the window installation, files or system.
2: Downloaded the latest Windows 10 ISO.
3: Mounted the ISO, let it get updates first and then did a Repair, selected keep all my files and settings (including installed applications and settings).
4: Perfect, the only program I had to reinstall was Acronis true image as it crashed if I tried to do a backup, reinstall of Acronis fixed the problem. I have a lot of programs and a programming environment, everything went back to normal, desktop resolution and all settings restored.
And it's now running with the latest problem update installed.
EDIT: My Administrator and default user accounts are now restored and working normally again in the users file system.
My take:
As I was digging into the USER accounts and security settings, it became apparent that even before the update, there was some inconsistencies in the system when compared to a second Win 10 PC.
I believe that over the past two years, the user account sections of my Windows 10 PRO 64 bit install had become compromised, either due to earlier window updates or during the trial of certain programs.
This may be why this update is causing problems with some users, an underlying glitch in their Windows system, one that does not show under normal Windows diagnostics.
EDIT: A repair is initiated by running setup from the Windows ISO. The wording seem to change in various articles, some call it a Repair / Reset but a Reset is different. When asked to download important updates before the Repair proceeds, do so. Before the repair starts there is a small clickable text area asking you to "Change what to keep" , select "Keep personal files apps and Windows settings".