I am confident that the problem is caused by a mandatory “gaming services” update.
The update replaces version 10.0.19041. 4332 with version 10.0.19041. 4271 and the sim won’t load.
There is no way to reverse this, unless you have a system restore point or c: drive backup that you can roll back to.
I restored an older backup of my OS drive (C:) and on bootup, I quickly paused MSStore from updating the gaming services app.
With automatic updates disabled in MStore, the Gaming Services update now waits for me to manually update (which I will not do until they fix the problem).
I have passed this info on to the ZenDesk.