Resolved/Fix below, from TheWindowsClub.com at least for my situation: 2] Clear WindowsImageBackup folder When you create a system image to some hard drive, all its content is stored in a WindowsImageBackup folder. It is possible that there are some leftovers of previous Windows Backup because of which you are not able to create a new backup and receive this error 0x80780166.
If this is the case, just access that hard drive on your computer where you want to create the backup. Look for the WindowsImageBackup folder at the root of the backup media and double-click on it. Select all the content available in that folder and delete it. After that, restart your computer, and then start the Windows Backup process. This might work.
I suspect the fix was the subdirectory in WindowsImageBackup folder was file name of my PC which was the same in three rebuilds from MS/CRWD and 2H 2024 Upgrades rebuilds. While the computer name is correct, I suspect the meta data for the name did not pass a secuity/audit check and thus kept failing (of course the prior fix was unlocking BitLocker target drive first).
