Cumulative Update KB5008212 will not install

Well, I saw your reply earlier while at work and was discouraged.
When I got home, I told the kids to leave me alone and immediately jumped back into the updates.
I cleared my SoftwareDisribution folder, restarted, and went to updates.

Instead of just the one update; I now had four.
As a software developer myself, I feel like an idiot for not recording which updates they were, and the associated KB number.
But, here are a few things I noticed:

1. One of the updates included a .NET Framework update. This was downloaded and installed before the Cumulative update, successfully.
2. I am unsure of the other two smaller updates; one was a Microsoft Defender update, I believe. Both of those downloaded and installed, successfully.
3. Then the Cumulative update began to download. But, the KB number was different. I'm very sorry I did not write it down. I am going to look in my logs after my computer restarts and see if I can find it there.

The Cumulative update downloaded, started installing, passed 20%, passed 44%, passed 76%, and FINALLY switched to "Pending Restart".
I am still waiting for the updates to install during the restart (I'm at 30% now), but it's looking like it worked.

NOTE:
One other thing I forgot to mention.
While I was browsing for solutions, I found someone who suggested going to "Advanced Options" on the Updates page, and enabling the "Download using metered connection". I figured, what the heck, give it a try. Prior, I noticed one of the four updates had failed. Once I enabled that option, it picked right back up and installed successfully. I'm not sure if this is what made everything download and install successfully, but I would ABSOLUTELY suggest you giving it a try!

Anxiously awaiting your response...

Wow!! I'm successfully "Windows is up to date"!
Never thought I would be so happy to see that little green checkbox.

So, here are the four updates that installed today:

1. HP Inc. - HIDCLass - 2.1.16.30156
2. Security Intelligence Update for Microsoft Defender Antivirus - KB2267602 (Version 1.355.1751.0 )
3. 2022-01 Cumulative Update for .NET Framework 3.5 and 4.8 for Windows 10 Version 20H2 for x64 (KB5008876)
4. 2022-01 Cumulative Update for Windows 10 Version 20H2 for x64-based Systems (KB5009543)

So, not sure why number 4 has a different KB number than the one I was trying to install yesterday. But again, all is working well now.
Hoping you get similar results.

Did KB5008212 install? That's what's been failing for me since Patch Tuesday in December and kept the updates of today from installing except for the defender definitions.

The update downloads just fine but fails at 20% during instillation including several times today. I have generated a Feedback Hub report complete with reproducing the update fail. Not sure it will do any good but it was worth a try.

Did you delete the entire Windows>Software Distribution folder or just the Download folder within? One fix I found said to delete the contents of the download folder but nothing else.

KB5008212 disappeared.
It never installed last night; and I shut down the computer.
Coming back today; the four updates I had as listed above.
It seems like my computer replaced KB5008212 with KB5009543.
They (from what I remember) are named VERY similarly.

Try enabling that metered connection option.
I seriously think that had something to do with my successful installations.
It did hang at 20% for about 20 to 30 minutes; but I just left it alone and it progressed (this is the KB5009543).

I deleted the entire Windows>SoftwareDistribution folder.
It seems it is repopulated every time Windows Update starts doing it's thing.

I had the same thing. It now shows in update history that KB5008212 installed successfully on 12/27/2021. But I've attempted to install the new update, KB5009543 five times. It gets all the way to installing - 100% and I get the error message (0x800f020b). I tried the metered connection too. :-(