You should be able to roll back 20H2. Have you tried that?
There’s a misunderstanding, 20H2 was never able to install, that’s when the troubles began. I’m still on Win 10 version 1909.
This seems familiar to me.
I don’t recall having all the diagnostics you describe, but I had a laptop on 21H1 that wouldn’t update. Attempts to force 20H2 would succeed to about the 55% mark and then fail and roll back.
I built a bootable 20H2 USB drive and successfully upgraded to 20H2 that way.
This will be my next step if the current attempt fails. Is it still possible to build a 20H2 boot USB drive with the Microsoft Media Creation Tool?
Yes. I used the MCT linked from the Microsoft page for upgrading to 20H2.
Nope, after uninstalling every driver I know that is dispensable, the update failed once more. Now taking @gnoitall advice and building a USB boot drive with version 20H2. Wish me luck!
ETA: shit, I just noticed that I haven’t got a USB drive big enough to build a boot drive. Lost the last one big enough some time ago. I have to wait till tomorrow and buy a new one. I’ll report back.
Honestly, 21H1 and 20H2 are basically identical, even under the hood. There’s basically no reason that a computer that can run one couldn’t run the other. So it wouldn’t hurt if you wind up with 21H1 instead.
I hadn’t realized that 20H2 hadn’t installed. That would go back to the BIOS possibly being an issue. At this point, I’d say to go ahead and try the USB install, but you definitely should check the laptop manufacturer’s website to see if there is a new BIOS version for the laptop’s specific model.
Well, I give up for now. I don’t know what I have done to the universe to deserve this, but this problem is the hardest I ever had to crack. So I bought me a 32 GB USB drive and created a boot drive with the Media Creation Tool and configured USB as first boot device in BIOS. What happened was that every time the system tried to boot from the drive, it crashed with a screen of wild chaotic patterns. I thought that maybe something went wrong when creating the boot drive, and repeated it, only to get the same results. Frustrated, I took BigT’s last advice to update the BIOS, but the system told me that the firmware was up-to-date. Now I’m out of ideas, and will bring back the laptop to my parents so that they can finish the sale of the Berlin apartment with it. The machine is still running, only that it doesn’t get updates, but with my parents’ online behavior, I don’t see much of a problem (my father’s 86th birthday will be next Tuesday and he really isn’t into porn anymore, if ever 
). When that’s all over, I will try to wipe everything and make a clean install from scratch, with a different USB drive, maybe the one I bought is somehow corrupted.
You’re on version 2004, right? That should still be getting updates until December. Did you at least try manually downloading the September Cumulative update for Windows 10 2004, and seeing if that worked?
Depending on whether you have the 32-bit version or the 64-bit version, it will be one of the top two downloads from this Microsoft Update Catalog link.
I have the 1909 version (64 bit). Every update I tried, failed. But I’ll try your links. But not today. I’m sick of it now, I’ve been working on this problem since Sunday for several hours each day, and I need a break. Do you know the feeling when you ferociously hate a certain machine? I’m at that point.
But many thanks to all who contributed, your help was good anyway. Maybe I’ve at least learned a few tricks from that episode.