For me the reason was having remains of a previous installation of Norton Power Eraser (NPE) already uninstalled years ago. In the newly created user.000 folder, I found a folder with a file "NPETraceSessionBoot.etl" which pointed me in the right direction.
This file is a debug output written by Windows WMI at each startup, blocking the proper profile usage. This debug was enabled by NPE though a registry key. I disabled the debug output by deleting the "NPETraceSession" registry key and all sub keys at:
[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\WMI\Autologger\NPETraceSession]
and also the NPE folder present at: [userdir]\AppDataLocal\NPE
After this, the update worked seamless.
Thank you! This process worked for me, since NPETraceSessionBoot.etl was the one file that was left in my original username folder after the 1909 upgrade attempt on Jan 17, 2020 created a username.000 profile.
I called and reported this to Microsoft Support. The tech seemed very knowledgeable, and I was assured that a ticket was being created to release a patch for this problem as soon as possible. It seems like most other debug trace loggers that cause the username.000 problem have been dealt with, but they hadn't yet included the Norton Power Eraser one in their patches.
Note that I left the detritus of my old Zune installation, files and registry keys, on my machine before upgrading to 1909. I used the latest version of the Windows 10 Update Assistant from the Microsoft Website, Windows10Upgrade9252.exe, on Jan 18, 2020. It was able to automatically delete the offending Zune files and trace logger registry keys, so the Zune .etl files did not cause a problem during upgrade. Microsoft has apparently dealt with the Zune issue.