Q: Stuck in an authorization loop after Windows 10 upgrade

See If iTunes repeatedly prompts you to authorize your computer - Apple Support.

tt2

Windows 10 ought to be similar. I've just found the relevant folder here.

Windows

1. Quit iTunes.
2. Press the Windows key or Ctrl + Esc.
3. Click the Start search (Windows 7) or magnifying glass icon (Windows 8).
4. In the search bar, type %ProgramData%, then press the Return key.
5. Show hidden files:Windows 8: Click View, then click Options. On the View tab, select Show hidden files, folders, and drives.Windows 7: Click Organize, then Folder and search options. On the View tab, select Show hidden files, folders, and drives.
   
   Windows 10: File > Change folder and search options, or tick the Hidden Files button in the tool bar.
   
   
6. Navigate to the SC Info folder.
7. Open the Apple Computer folder, then the iTunes folder.
8. Right-click the SC Info folder and choose Delete from the pop-up menu.
9. Restart the computer.
10. Open iTunes.
11. Authorize your computer. If you have multiple accounts, authorize your computer for each account.

tt2

You're welcome.

tt2

If iTunes repeatedly prompts you to authorize your computer - Apple Support