Now KB5015814 won't install

Others have reported the same problem too. A repair install usually solves the problem.

Repair Install Windows 11 using an ISO file - Your Windows Guide

A repair install, also known as an in-place upgrade, allows you to reinstall Windows, keeping apps files and settings. It can fix a wide variety of issues, including Windows Update errors, store app issues that include downloads and updates, or unreliable windows like slowness or crashing. A repair install is highly successful in fixing these problems without the hassle of doing a reset that removes your existing applications.

First of all, download a windows 11 insider ISO file from the following link:
https://www.microsoft.com/en-in/software-download/Windows11

• Under Download ISO file, select Windows 11.
• Click Download and choose the Language in which Windows is installed.
• Click 64-bit Download to start the download.
• When the file has completed downloading, open it, which should mount the installation file to a separate drive in the File Explorer.
• Open the drive and click on setup.exe, which should show you a preparing screen.
• If you have trouble installing windows updates, click Change how setup downloads updates and skip checking for updates.
• The next step should confirm that your files, apps. Settings are kept. It would also show you the edition of windows it would install.
• When complete, you should see the login screen. Enter your PIN.
• The setup would give the final finishing touches, and you should see the desktop ultimately.

Thats what I did in the end.

Success at last.

I had this same experience. I posted what worked for me.

These steps worked for me, assuming you run as Admin or are Admin:

1. Uninstall MalwareBytes (if applicable)
2. Disable All Microsoft Defender Protections through Windows Security
3. Edit Group Policy (WIN-R, gpedit.msc) Browse to Local Computer Policy, Computer Configuration, Administrative Templates, Windows Components, Microsoft Defender Antivirus Double Click on Turn Off Microsoft Defender Antivirus, then select Enabled, and hit apply.
4. Reboot
5. Repeat steps 2 and 3, then try updating again
6. After Update, undo steps 1-3: Install MalwareBytes (if applicable) Edit Group Policy (WIN-R, gpedit.msc) Browse to Local Computer Policy, Computer Configuration, Administrative Templates, Windows Components, Microsoft Defender Antivirus Double Click on Turn Off Microsoft Defender Antivirus, select Not Configured, and hit apply. Enable All Microsoft Defender Protections through Windows Security Reboot

.Net Framework 3.5 (includes .Net 2.0 and 3.0) is already off when I accessed the menu. I'm using .Net Framework 4.8, which has WCF services and TCP Port Sharing on. Whatever those are.

BTW, I'm using Windows 10 but I came here because I saw that there a similar issue with Win11 and hoped that I can find a solution.

Also, there was a time (around mid-April) when "Asus Core SDK" in Armoury Crate couldn't install updates and the proposed fix for that was to install an Aura Wallpaper which forces an update, which worked when I tried at the end of April. Could that be a factor? Because the timing is just too coincidental as my Windows update problems started in May. It doesn't help that there apparently a fix for that in May, meaning that all I really had to do was wait.