Another Windows 10 Update Problem

Hiya,

I've been trying to update one of my computers. It's currently running Windows 10 Home 64 bit version 1909 (build 18363.1556 if that's important). When I go to Windows Update it says 'your device is missing important security and quality fixes' when I check for updates there isn't any.

Things I've tried:

Windows Update Troubleshooter - doesn't find anything
sfc /scannow - no errors
dism to scan and restore image etc - no problems
updated BIOS
updated drivers
installed servicing stack update
unplugged all peripherals
downloaded ISO image (of both version 2004 and the latest 21H1)
done all of the stop wuauserv, BITS, cryptserv etc renamed software distribution and catroot files
uninstalled updates
set the related services to automatic

when I try installing from ISO it say's that it couldn't finish the update and has reverted back and the error code is
'0xC1900101 - 0x20017 - the instillation failed in the SAFE_OS phase with an error during BOOT operation'

I have also had an error 0x80240017 to do with a Brother printer (drivers I think) I have updated drivers for the printer.
I've used the wushowhide to hide it from Windows Update.

I would try a clean boot but if Windows Update isn't finding any updates I don't see the point.

Not sure what else to try, I really don't want to reinstall Windows from scratch.

Any other ideas are more than welcome!

Thanks both :)

I've never heard of the safeguard thing, maybe that's the problem. I'll post the result from setupdiag.exe when I am back at home.

Somehow the PC updated this last time that I ran the ISO, after weeks of trying!

Thanks guys!