Error 0xc000021a when booting Windows 7, cannot install Windows 10

The situation is as follows:

A Windows 7 machine whose PSU was replaced not too long ago has started failing to boot, with startup repair, sfc and dism and anything similar doing nothing.

However, manually launching the PC with "Disable driver signature enforcement" allows machine to boot normally.

Since the machine was due for a Windows 10 upgrade, I figured I should ignore the likely software issue and start from scratch, but Windows 10 install (from a USB stick) also fails to install with error code 0xc000021a or even 0x8007025d.

In the end it turns out it was a hardware issue, perhaps even linked to the PSU failure.

With a quick memory test using Memtest86+, it turned out the RAM stick was corrupt 3220 MB in (out of 4GB) (note that some RAM issues may not show immediately on first pass!).

Since the machine was mostly used for document viewing, no issues appeared during everyday use.

Replacing the RAM stick fixes the issue.