I have done the clean boot. Problem persists.
I have even done a clean Windows install on one machine. Problem persists.
I have created a new user. Problem persists.
The two machines are not similar in settings / apps as they are being used for different purposes.
There are two common features, though. They both use Start 10 and they both use Intel graphics.
The event viewer shows an explorer.exe crash:
Faulting application path: C:\WINDOWS\Explorer.EXE
Faulting module path: C:\WINDOWS\SYSTEM32\ntdll.dll
I installed the competing product "Start is Back" - NO problem
So, there must be something specifically wrong with Start10 (which I prefer...!)