Yes I have the same issue. 400G2 Prodesk
Can't seem to get the older version of BIOS back either.
I read I should hold the windows and B keys down when switching on - but nothing happens goes straight to beep 512 Rear fan error.
Also I tried to see if I could reload a previous version of BIOS but according to the HP website ...
Added improved Security Updates.
*** ALERT ***
This release improved Security Updates implementation. HP strongly recommends promptly
transitioning to this BIOS version. Due to security changes, after this BIOS
update has been installed, previous BIOS versions cannot be reinstalled.
So you cant go back to the old version - and the new version doesnt work. Nice one HP.