Finally got it sorted out…
Uninstall of MSFS did not work either - “Uninstall” made the ‘Install’ button available again, but the reinstall always failed with a 0x80070424 error. I was ready to give up and reinstall everything (my whole machine) from scratch when I came across this post:
It’s basically just
get-appxpackage Microsoft.GamingServices | remove-AppxPackage -allusers
start ms-windows-store://pdp/?productid=9MWPM2CQNLHN
then reinstall the Gaming Services, and then I was able to install MSFS again. Phew.
Lesson learnt, I will now make a backup of my Win system partition every week.