This sounds like another instance of this, though that questioner never came back and reported results; and maybe also this that was posted at about the same time as yours.

Based on your and @Oz-M 's reports, I think that the update to Windows 10 ver 20H2 changed the permissions in the C:\Program Files hierarchy, meaning that your normal user account doesn’t have write permission to that directory hierarchy anymore.

Could you follow my advice from that post:

Make sure that you have proper write-permissions in the C:\Program Files\Notepad++\ and C:\Program Files\Notepad++\functionList\ directories. You might have to run the installer in Run as Administrator mode to get Windows to give you those permissions, or use Windows Explorer’s right click Properties > Security > Edit (or Advanced) to give your normal user permission on those directories.

Also, make sure that when you run the installer, you don’t have another instance of Notepad++ running, and that no other application has c.xml open. (To make sure this is true, you may want to reboot before running the installer.)