Microsoft has broken something in their backup software

Error returned while creating the volume shadow copy:80042301

Not clear what's happening here, but sometime in the past few months it seems a windows upgrade broke the shadow copy mode. Regular backup mode works, but one can see this errror message in the log.

This MCSE note says:

If you're using an NTFS volume, try the steps listed below:

Open Regedt32.exe and navigate to the following key:

HKEY_LOCAL_MACHINE\system\setup

Change the "UpgradeInProgress" value to 0.

Unfortunately, there's no such key in my registry. The closest one is "SystemSetupInProgress" and it's already set to 0.