There have been a number of reports that KB4532920 is causing endless reboots loops on Windows Server 2012. One of my customers experienced this today. This update was released on December 10th 2019.
This is a servicing stack update which improves the updates process on Windows, ironic that installing it puts the server into an endless reboot loop!
I was able to work around the problem by booting into safe mode with command prompt and then issuing the command:
shutdown /r /t 1
The problem was that I was unable to access safe mode because by default the F8 boot options are disabled on Windows Server 2012.
To access safe mode in Windows Server 2012
- Boot from the Windows Server ISO.
- Choose “Repair your computer” from the main installation dialog:
- On the next screen choose “Troubleshoot”.
- Then choose command prompt.
- At the command prompt enter these commands:
- bcdedit /set {bootmgr} displaybootmenu yes
- bcdedit /set {bootmgr} timeout 10
Now reboot the machine and you should be able to access the boot options. Boot into safe mode and login as normal. At the command prompt you can just reboot the machine again. This particular machine required another reboot before it came up normally. But the update did show as having been installed:

These unexpected problems really waste a lot of time, but thankfully it was not too difficult to fix.