Windows backup says device is not ready; Backup & Recovery Community Edition

symptom:
The standard backup of Windows cannot finish because "The device is not ready."

remedy:
Restore the disk image from the backup by the standard Windows backup function.

environment:
Windows 10 HOME, 21H2, build 19044.1620.
Paragon Backup & Recovery Community Edition, 17.9.3.4927.

date:
2022 March 26.

sequence:
Installed B&R 17 CE.
Tried backup Windows from C-drive to E-drive.
(Usually I takes backup C-drive to G-drive.)
Mostly succeeded, but once failed.
Deleted the failed backup within the GUI of 17 CE - OK, it seemed.
I did not delete the backup plan, only deleted the failed instance.
The 17 CE (from C to E) and Windows backup (from C to G, Windows 7 style using the old Control Panel) both failed.
The Windows backup said "The device is not ready(0x80070015)."
Then I tried to make a recovery USB with system, and Windows said "it cannot make system because necessary files are missing."
"vssadmin list providers" seemed ok, "vssadmin list writers" reported no error.
Following actions failed to resolve the problem - 1)chkdsk, 2)dism, 3)sfc, 4)reboot the PC, 5)restore Windows to where 17 CE succeeded making backup, 6)remove all restore points and shadow copies.
I restored Windows just after installed 17 CE, and resolved.
I uninstalled 17 CE.

afterthought:
I suspect some removal/clearing procedure of 17 CE destroyed vss system or something.
I have no courage to reproduce this disaster.

Goodbye and Good Luck!