Fixing ACPI_BIOS_ERROR after a BIOS upgrade

Just posting a problem and its solution here in case it might help others:

I have an old Twin Xeon Dell Precision T5400.
I've extended its life a bit with a SSD, modern graphics card and Windows 10.
Last week the Dell Support Assistant popped up and asked to perform a few optimizations which included updating the BIOS (from A04 TO A11) which, perhaps foolishly, I allowed.
After rebooting I could only get a BSOD with 'Something went wrong... ACPI_BIOS_ERROR'.
I spent many hours attempting to recover/re-install Windows but always ended in failure (and a badly nuked SSD).
Long story short - the solution was to roll back the BIOS.
Dell helpfully have old BIOS installers available on the support website so, using a working PC, I grabbed a copy of T5400A04.exe. I prepared a FreeDOS bootable USB stick using Rufus (search 'rufus freedos') and put T5400A04.exe onto my FreeDOS USB stick.
I booted from USB, ran the T5400A04.exe BIOS installer, and rebooted the PC.
Voila, this time I could successfully install Windows 10.
Out of curiosity I went back and grabbed a copy of T5400A11.exe to 'upgrade' the BIOS again to see if the problem returned to prove to my satisfaction that it wasn't a chance fluke. The ACPI_BOIS_ERROR did re-appear so I quickly installed A04 again.

If I had known this before all the failed recovery/re-installation attempts I might not have had to re-install all my application software. Should have been more careful, but such is life...